A humidifying system for a fuel cell includes: a fuel cell having an anode and a cathode, the anode being supplied with a fuel gas and the cathode being supplied with an oxidant gas so that the fuel gas and the oxidant gas chemically react within the fuel cell to generate electricity; a first humidifier transferring moisture of cathode exhaust gas discharged from the cathode of the fuel cell to the fuel gas through hollow fiber membranes; a second humidifier transferring moisture of cathode exhaust gas discharged from the first humidifier to the oxidant gas through hollow fiber membranes; and a reduced pressure generating device arranged downstream of the first humidifier and between the first humidifier and the fuel cell to mix part of anode exhaust gas discharged from the anode of the fuel cell with the fuel gas using negative pressure resulting from a flow of the fuel gas.