diff --git a/atcoder/internal_math.hpp b/atcoder/internal_math.hpp
index 2f45a70..fc71d5a 100644
--- a/atcoder/internal_math.hpp
+++ b/atcoder/internal_math.hpp
@@ -177,6 +177,35 @@ constexpr int primitive_root_constexpr(int m) {
 }
 template <int m> constexpr int primitive_root = primitive_root_constexpr(m);
 
+// @param n `n < 2^32`
+// @param m `1 <= m < 2^32`
+// @return sum_{i=0}^{n-1} floor((ai + b) / m) (mod 2^64)
+unsigned long long floor_sum_unsigned(unsigned long long n,
+                                      unsigned long long m,
+                                      unsigned long long a,
+                                      unsigned long long b) {
+    unsigned long long ans = 0;
+    while (true) {
+        if (a >= m) {
+            ans += n * (n - 1) / 2 * (a / m);
+            a %= m;
+        }
+        if (b >= m) {
+            ans += n * (b / m);
+            b %= m;
+        }
+
+        unsigned long long y_max = a * n + b;
+        if (y_max < m) break;
+        // y_max < m * (n + 1)
+        // floor(y_max / m) <= n
+        n = (unsigned long long)(y_max / m);
+        b = (unsigned long long)(y_max % m);
+        std::swap(m, a);
+    }
+    return ans;
+}
+
 }  // namespace internal
 
 }  // namespace atcoder
diff --git a/atcoder/math.hpp b/atcoder/math.hpp
index 0b6e366..5de8736 100644
--- a/atcoder/math.hpp
+++ b/atcoder/math.hpp
@@ -80,20 +80,20 @@ std::pair<long long, long long> crt(const std::vector<long long>& r,
 }
 
 long long floor_sum(long long n, long long m, long long a, long long b) {
-    long long ans = 0;
-    if (a >= m) {
-        ans += (n - 1) * n / 2 * (a / m);
-        a %= m;
+    assert(0 <= n && n < (1LL << 32));
+    assert(1 <= m && m < (1LL << 32));
+    unsigned long long ans = 0;
+    if (a < 0) {
+        unsigned long long a2 = internal::safe_mod(a, m);
+        ans -= 1ULL * n * (n - 1) / 2 * ((a2 - a) / m);
+        a = a2;
     }
-    if (b >= m) {
-        ans += n * (b / m);
-        b %= m;
+    if (b < 0) {
+        unsigned long long b2 = internal::safe_mod(b, m);
+        ans -= 1ULL * n * ((b2 - b) / m);
+        b = b2;
     }
-
-    long long y_max = a * n + b;
-    if (y_max < m) return ans;
-    ans += floor_sum(y_max / m, a, m, y_max % m);
-    return ans;
+    return ans + internal::floor_sum_unsigned(n, m, a, b);
 }
 
 }  // namespace atcoder
diff --git a/document_en/math.md b/document_en/math.md
index 8a51537..6154802 100644
--- a/document_en/math.md
+++ b/document_en/math.md
@@ -65,17 +65,20 @@ $y, z$ $(0 \leq y < z = \mathrm{lcm}(m[i]))$. It returns this $(y, z)$ as a pair
 ll floor_sum(ll n, ll m, ll a, ll b)
 ```
 
-It returns $\sum_{i = 0}^{n - 1} \mathrm{floor}(\frac{a \times i + b}{m})$.
+It returns
+
+$$\sum_{i = 0}^{n - 1} \left\lfloor \frac{a \times i + b}{m} \right\rfloor$$
+
+It returns the answer in $\bmod 2^{\mathrm{64}}$, if overflowed.
 
 **@{keyword.constraints}**
 
-- $0 \leq n \leq 10^9$
-- $1 \leq m \leq 10^9$
-- $0 \leq a, b \lt m$
+- $0 \leq n \lt 2^{32}$
+- $1 \leq m \lt 2^{32}$
 
 **@{keyword.complexity}**
 
-- $O(\log{(n+m+a+b)})$
+- $O(\log{(m+a)})$
 
 ## @{keyword.examples}
 
diff --git a/document_ja/math.md b/document_ja/math.md
index db116de..7fa4caa 100644
--- a/document_ja/math.md
+++ b/document_ja/math.md
@@ -64,19 +64,18 @@ $$x \equiv r[i] \pmod{m[i]}, \forall i \in \lbrace 0,1,\cdots, n - 1 \rbrace$$
 ll floor_sum(ll n, ll m, ll a, ll b)
 ```
 
-$\sum_{i = 0}^{n - 1} \mathrm{floor}(\frac{a \times i + b}{m})$
+$$\sum_{i = 0}^{n - 1} \left\lfloor \frac{a \times i + b}{m} \right\rfloor$$
 
-を返します。
+を返します。答えがオーバーフローしたならば $\bmod 2^{\mathrm{64}}$ で等しい値を返します。
 
 **@{keyword.constraints}**
 
-- $0 \leq n \leq 10^9$
-- $1 \leq m \leq 10^9$
-- $0 \leq a, b \lt m$
+- $0 \leq n \lt 2^{32}$
+- $1 \leq m \lt 2^{32}$
 
 **@{keyword.complexity}**
 
-- $O(\log{(n+m+a+b)})$
+- $O(\log{(m+a)})$
 
 ## @{keyword.examples}
 
diff --git a/test/unittest/math_test.cpp b/test/unittest/math_test.cpp
index c2d80a9..8cb2ee7 100644
--- a/test/unittest/math_test.cpp
+++ b/test/unittest/math_test.cpp
@@ -27,7 +27,8 @@ ll pow_mod_naive(ll x, ull n, uint mod) {
 ll floor_sum_naive(ll n, ll m, ll a, ll b) {
     ll sum = 0;
     for (ll i = 0; i < n; i++) {
-        sum += (a * i + b) / m;
+        ll z = a * i + b;
+        sum += (z - internal::safe_mod(z, m)) / m;
     }
     return sum;
 }
@@ -93,8 +94,8 @@ TEST(MathTest, InvModZero) {
 TEST(MathTest, FloorSum) {
     for (int n = 0; n < 20; n++) {
         for (int m = 1; m < 20; m++) {
-            for (int a = 0; a < 20; a++) {
-                for (int b = 0; b < 20; b++) {
+            for (int a = -20; a < 20; a++) {
+                for (int b = -20; b < 20; b++) {
                     ASSERT_EQ(floor_sum_naive(n, m, a, b),
                               floor_sum(n, m, a, b));
                 }