diff --git a/lib/bigdecimal/math.rb b/lib/bigdecimal/math.rb
index 3d96e150..b264c8b8 100644
--- a/lib/bigdecimal/math.rb
+++ b/lib/bigdecimal/math.rb
@@ -85,6 +85,7 @@ def sin(x, prec)
       d   = sign * x1.div(z,m)
       y  += d
     end
+    y = BigDecimal("1") if y > 1
     neg ? -y : y
   end
 
@@ -130,7 +131,7 @@ def cos(x, prec)
       d   = sign * x1.div(z,m)
       y  += d
     end
-    y
+    y < -1 ? BigDecimal("-1") : y > 1 ? BigDecimal("1") : y
   end
 
   # call-seq:
diff --git a/test/bigdecimal/test_bigmath.rb b/test/bigdecimal/test_bigmath.rb
index 545a963e..e3402211 100644
--- a/test/bigdecimal/test_bigmath.rb
+++ b/test/bigdecimal/test_bigmath.rb
@@ -53,6 +53,8 @@ def test_sin
     assert_fixed_point_precision {|n| sin(BigDecimal("1e-30"), n) }
     assert_fixed_point_precision {|n| sin(BigDecimal(PI(50)), n) }
     assert_fixed_point_precision {|n| sin(BigDecimal(PI(50) * 100), n) }
+    assert_operator(sin(PI(30) / 2, 30), :<=, 1)
+    assert_operator(sin(-PI(30) / 2, 30), :>=, -1)
   end
 
   def test_cos
@@ -74,6 +76,8 @@ def test_cos
     assert_fixed_point_precision {|n| cos(BigDecimal("1e50"), n) }
     assert_fixed_point_precision {|n| cos(BigDecimal(PI(50) / 2), n) }
     assert_fixed_point_precision {|n| cos(BigDecimal(PI(50) * 201 / 2), n) }
+    assert_operator(cos(PI(30), 30), :>=, -1)
+    assert_operator(cos(PI(30) * 2, 30), :<=, 1)
   end
 
   def test_atan