NOVELTY - The reformer comprises a reformer main body, a slot formed in an outer surface of the reformer main body, and an internal space for reforming fuel. A reforming reaction of a reforming fuel takes place in the internal space using a reforming catalyst in a catalyst unit. A cartridge part includes a catalyst cartridge set up in the slot of the reformer main body. The cartridge part corresponds to the internal space. The reformer main body includes a support groove that is formed for the insertion of the catalyst cartridge inserted through the slot and supported in the internal space.    USE - The reformer is useful for a fuel cell system of a ship (claimed).    ADVANTAGE - The reformer has stable operation and reduces the need of maintenance.    DETAILED DESCRIPTION - The reformer comprises a reformer main body, a slot formed in an outer surface of the reformer main body, and an internal space for reforming fuel. A reforming reaction of a reforming fuel takes place in the internal space using a reforming catalyst in a catalyst unit. A cartridge part includes a catalyst cartridge set up in the slot of the reformer main body. The cartridge part corresponds to the internal space of the reformer main body. The reformer main body includes: a support groove that is formed for the insertion of the catalyst cartridge inserted through the slot and supported in the internal space; and a cover formed in order to open and close the slot. The reformer further comprises: a seal member for sealing a contact gap of the catalyst cartridge and the slot; an elastic member that is installed in order to push the seal member by an elastic force and flexibly blocks the contact gap; and a poisoning sensor including a thermometer or a sulfur detector and for sensing the poisoning of the reforming catalyst in the reformer main body. The catalyst is metallic catalyst or ceramic catalyst. The poisoning sensor senses the temperature or sulfur material of a spot corresponding to a holding fixture in the internal space of the reformer main body.