NOVELTY - A hydrogen generation device comprises reformers (10A, 10B, 20A, 20B), each of which includes a catalyst; switch (V1-V12, V15-V25) for switching among supply passages through which the reactant is supplied to the reformers, and among supply passages through which exothermic material is supplied to the reformers; and switch control for controlling the switch so that at least one reformer in which a temperature of the catalyst reaches a reforming-start temperature at which the reforming reaction can be carried out is selected among the reformers.    USE - The hydrogen generation device is used in fuel cell system (claimed).    ADVANTAGE - A reforming reaction for generating hydrogen can be quickly started, and the required amount of hydrogen can be quickly and efficiently generated by the reforming reaction. The required amount of electric power can also be stably generated.    DETAILED DESCRIPTION - A hydrogen generation device comprises:    (1) reformers, each of which includes a catalyst, and in each of which a reforming reaction is carried out using reactant on the catalyst, and an exothermic reaction is carried out using exothermic material to heat the catalyst;    (2) switch for switching among supply passages through which the reactant is supplied to the reformers, and switching among supply passages through which the exothermic material is supplied to the reformers; and    (3) switch control for controlling the switch so that at least one reformer in which a temperature of the catalyst reaches a reforming-start temperature at which the reforming reaction can be carried out is selected among the reformers, and the reactant is supplied to the selected reformer, in a case where the reforming reaction is started.    INDEPENDENT CLAIMS are also included for:    (1) fuel cell system comprising the hydrogen generation device; and fuel cell that generates electric power using hydrogen-containing gas that is generated by the hydrogen generation device; and    (2) controlling a hydrogen generation device comprising determining whether a temperature of a catalyst in each reformer has reached a reforming-start temperature at which a reforming reaction can be carried out; selecting at least one reformer in which the temperature of the catalyst has reached the reforming-start temperature; and supplying reactant to the selected reformer.    DESCRIPTION OF DRAWING(S) - The figure is a schematic diagram showing the configuration of a fuel cell system.    Reformers (10A, 10B, 20A, 20B)    Switches (V1-V12, V15-V25)