A humidifier for a fuel cell enables moisture exchange between supply air supplied from an air compressor and exhaust air discharged from the fuel cell. In particular, the humidifier of the fuel cell may include: i) a housing main body; ii) a humidification membrane module that is provided in the housing main body; and iii) a bypass unit that is provided in the housing main body to selectively bypass the supply air supplied from the air compressor and the exhaust air discharged from the fuel cell.