A humidifying device, which exchanges moisture between air supplied from an air compressor and air exhausted from the fuel cell, for a fuel cell, includes: a housing main body; at least one humidifying membrane module disposed in the housing main body; and a valve assembly which is disposed in the housing main body and adjusts a supply amount of exhaust air flowed into the at least one humidifying membrane module based on a predetermined supply amount of supply air flowed into the at least one humidifying membrane module.