A humidifier and a method for producing it can reduce gas leakage through inside the surfaces of humidifying membranes or through interfaces between the humidifying membranes and separators. The humidifier includes water permeable humidifying membranes and gas separators each having one or two channels opened at least one side in a direction of lamination, through which at least one of a dry gas and a wet gas is caused to flow. A humidifying membrane, a gas separator, a humidifying membrane and a gas separator are repeatedly laminated one over another in this order, or a gas separator, a humidifying membrane and a gas separator are repeatedly laminated one over another in this order. Each gas separator has a frame-shaped portion surrounding the one or two channels, and that portion of each humidifying membrane which, when laminated, faces the frame-shaped portion of a corresponding gas separator is filled with a resin.