A humidifier for a fuel cell includes a body, first and second humidifying spaces formed inside the body, an exhaust gas inlet and an exhaust gas outlet for supplying exhaust gas released from the fuel cell stack into the first and second humidifying spaces, a passing space formed inside the body and directly or indirectly communicated with the second humidifying space and the fuel cell stack. The inflow gas flows into the passing space from the first humidifying space. A valve is installed in the passing space to allow the inflow gas introduced into the passing space to flow into the fuel cell stack with or without passing through or to allow some of the inflow gas introduced into the passing space to flow into the fuel cell stack passing through the second humidifying space and others of the inflow gas introduced into the fuel cell stack without passing through the second humidifying space.