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; and a first cap coupled to one end of the humidifying module, wherein the humidifying module comprises: a mid-case and at least one cartridge disposed in the mid-case, the cartridge being configured to receive a plurality of hollow fiber membranes, the humidifier further comprises a first packing member airtightly coupled to at least one end of the humidifying module through mechanical assembly such that the first cap fluidly communicates with only the hollow fiber membranes, the first packing member comprises: a first soft member configured to contact each of the cartridge and the mid-case; and a first hard member coupled to the first soft member, the first soft member comprises a first insertion groove configured to allow the first hard member to be inserted thereinto, the first hard member is deformed so as to extend in a separation direction in which the cartridge and the mid-case are separated from each other in a state of being inserted into the first insertion groove, whereby the first soft member is brought into tight contact with the cartridge, wherein the first hard member is plastically deformed so as to extend in the separation direction, whereby the first hard member is maintained in a state in which the first soft member is in tight contact with the cartridge, and wherein the first hard member comprises: a first pressing member disposed so as to face the cartridge; a first outer wall member disposed so as to face the mid-case; a first connection member configured to connect the first pressing member and the first outer wall member to each other; and a first extension groove disposed between the first pressing member and the first outer wall member. a humidifying module configured to humidify dry gas supplied from outside using wet gas discharged from a fuel cell stack; and a first cap coupled to one end of the humidifying module, wherein the humidifying module comprises: a mid-case and at least one cartridge disposed in the mid-case, the cartridge being configured to receive a plurality of hollow fiber membranes, the humidifier further comprises a first packing member airtightly coupled to at least one end of the humidifying module through mechanical assembly such that the first cap fluidly communicates with only the hollow fiber membranes, the first packing member comprises: a first soft member configured to contact each of the cartridge and the mid-case; and a first hard member coupled to the first soft member, the first soft member comprises a first insertion groove configured to allow the first hard member to be inserted thereinto, the first hard member is deformed so as to extend in a separation direction in which the cartridge and the mid-case are separated from each other in a state of being inserted into the first insertion groove, whereby the first soft member is brought into tight contact with the cartridge, wherein the first hard member is plastically deformed so as to extend in the separation direction, whereby the first hard member is maintained in a state in which the first soft member is in tight contact with the cartridge, and wherein the first hard member comprises: a first pressing member disposed so as to face the cartridge; a first outer wall member disposed so as to face the mid-case; a first connection member configured to connect the first pressing member and the first outer wall member to each other; and a first extension groove disposed between the first pressing member and the first outer wall member. a first pressing member disposed so as to face the cartridge; a first outer wall member disposed so as to face the mid-case; a first connection member configured to connect the first pressing member and the first outer wall member to each other; and a first extension groove disposed between the first pressing member and the first outer wall member.