A humidifier for fuel cells, the humidifier comprising: a humidifying module configured to humidify dry gas supplied from outside using wet gas discharged from a fuel cell stack; a first cap coupled to one end of the humidifying module; and a second cap coupled to the other end of the humidifying module, wherein the humidifying module comprises: a cartridge, and a mid-case, wherein cartridge comprises: an inner case configured to be inserted into the mid-case of a humidifier for fuel cells configured to humidify dry gas supplied from outside using wet gas discharged from a fuel cell stack; a plurality of hollow fiber membranes received in the inner case; and an interlocking member configured to connect the inner case to the mid-case such that the mid-case and the inner case are moved in an interlocked state depending on pressure of wet gas located in the mid-case, wherein the cartridge is moved in an insertion direction so as to be disposed in the mid-case and is moved in a separation direction opposite the insertion direction so as to be separated from the mid-case, wherein the mid-case comprises: an inlet configured to allow wet gas to be introduced therethrough; an outlet configured to allow the wet gas to be discharged therethrough; and a blocking member protruding toward the cartridge between the inlet and the outlet, the blocking member being configured to contact the cartridge in order to block passage of wet gas, wherein the blocking member comprises a catching groove configured to allow the interlocking member to be inserted thereinto, and wherein the interlocking member comprises: a coupling member protrudes from the outer surface of the inner case; and a catching member protrudes from the coupling member in the insertion direction, wherein the catching member is configured to be inserted into the catching groove; and the coupling member is coupled to each of the catching member and the inner case such that the inner case is moved together as the catching member is moved while being interlocked with the blocking member in a state in which the catching member is inserted into the catching groove, wherein the interlocking member connects the inner case to the blocking member such that the blocking member and the inner case are moved in an interlocked state depending on pressure of wet gas located between the mid-case and the inner case. a humidifying module configured to humidify dry gas supplied from outside using wet gas discharged from a fuel cell stack; a first cap coupled to one end of the humidifying module; and a second cap coupled to the other end of the humidifying module, wherein the humidifying module comprises: a cartridge, and a mid-case, wherein cartridge comprises: an inner case configured to be inserted into the mid-case of a humidifier for fuel cells configured to humidify dry gas supplied from outside using wet gas discharged from a fuel cell stack; a plurality of hollow fiber membranes received in the inner case; and an interlocking member configured to connect the inner case to the mid-case such that the mid-case and the inner case are moved in an interlocked state depending on pressure of wet gas located in the mid-case, an inner case configured to be inserted into the mid-case of a humidifier for fuel cells configured to humidify dry gas supplied from outside using wet gas discharged from a fuel cell stack; a plurality of hollow fiber membranes received in the inner case; and an interlocking member configured to connect the inner case to the mid-case such that the mid-case and the inner case are moved in an interlocked state depending on pressure of wet gas located in the mid-case, wherein the cartridge is moved in an insertion direction so as to be disposed in the mid-case and is moved in a separation direction opposite the insertion direction so as to be separated from the mid-case, wherein the mid-case comprises: an inlet configured to allow wet gas to be introduced therethrough; an outlet configured to allow the wet gas to be discharged therethrough; and a blocking member protruding toward the cartridge between the inlet and the outlet, the blocking member being configured to contact the cartridge in order to block passage of wet gas, an inlet configured to allow wet gas to be introduced therethrough; an outlet configured to allow the wet gas to be discharged therethrough; and a blocking member protruding toward the cartridge between the inlet and the outlet, the blocking member being configured to contact the cartridge in order to block passage of wet gas, wherein the blocking member comprises a catching groove configured to allow the interlocking member to be inserted thereinto, and wherein the interlocking member comprises: a coupling member protrudes from the outer surface of the inner case; and a catching member protrudes from the coupling member in the insertion direction, a coupling member protrudes from the outer surface of the inner case; and a catching member protrudes from the coupling member in the insertion direction, wherein the catching member is configured to be inserted into the catching groove; and the coupling member is coupled to each of the catching member and the inner case such that the inner case is moved together as the catching member is moved while being interlocked with the blocking member in a state in which the catching member is inserted into the catching groove, wherein the interlocking member connects the inner case to the blocking member such that the blocking member and the inner case are moved in an interlocked state depending on pressure of wet gas located between the mid-case and the inner case.