A check valve includes a main-casing, a sub-casing, a positive-pressure valve and a negative-pressure valve. The positive-pressure valve has through holes and a negative-pressure valve seat. The negative-pressure valve has leg portions. A cap is disposed within the positive-pressure valve. A positive-pressure spring is disposed between the positive-pressure valve and the sub-casing, and a negative-pressure spring is disposed between the positive-pressure valve and the cap. A positive-pressure flow path for releasing fuel vapor is formed between an outer circumference of the positive-pressure valve and an inner circumference of the main-casing, at further outward position than the positive-pressure spring. A negative-pressure flow path for introducing outside air is formed between an inner circumference of the positive-pressure valve and the leg portions and an outer circumference of the cap and between the through holes and the leg portions, at further outward position than the negative-pressure spring.