A humidifier for a fuel cell is provided and includes a housing having, at a first side, a moist air supply port through which moist air is supplied from a fuel cell stack and having, at a second side, a moist air discharge port. A humidifying membrane is disposed in the housing and allows dry air to flow along the inside of the humidifying membrane. A bypass flow path is formed in the housing to allow a part of the moist air supplied to the moist air supply port to continuously flow to the moist air discharge port without passing through the humidifying membrane, thereby adjusting the amount of humidification and a differential pressure of the humidifier based on an operating condition of a fuel cell.