In a humidifier including a hollow fiber membrane bundle housed in a case, the hollow fiber membranes exchange water therethrough between the first and second gases flowing inside and outside the hollow fiber membrane to humidify one of the first and second gases having the water content lower than the other, and an annular mesh member throttles a part near the gas outlet of the second gas to provide a space between the part and the gas outlet of the second gas to easily discharge the second gas.