NOVELTY - A hydrogen generating apparatus (100A) has a reformer (1) having a reforming catalyst (1a), a source gas supply device (13), an evaporator (3), a water supply machine, and a controller (10) for controlling source gas supply device and water supply machine so as to terminate supply of source gas to reformer and supply of water to evaporator, and subsequently controlling source gas supply device during a period in which residual water is evaporated in evaporator and/or reformer so as to supply source gas to the reformer.    USE - Hydrogen generating apparatus is used for fuel cell system (claimed) used as domestic power supply.    ADVANTAGE - The water vapor oxidation of reforming catalyst is reduced, and deterioration of reforming catalyst is prevented.    DETAILED DESCRIPTION - A hydrogen generating apparatus has a reformer having a reforming catalyst for producing hydrogen-containing gas by performing reforming reaction using a source gas and water vapor, a source gas supply device for adjusting supply amount of source gas to modifier, an evaporator for generating water vapor and supplying water vapor to modifier, a water supply machine for adjusting supply amount of water to evaporator, and a controller for controlling source gas supply device and water supply machine so as to terminate supply of source gas to reformer and supply of water to evaporator, and subsequently controlling source gas supply device during a period in which residual water is evaporated in evaporator and/or reformer so as to supply source gas to the reformer. INDEPENDENT CLAIMS are included for the following:    (1) fuel cell system; and    (2) operating method of hydrogen generating apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of hydrogen generating apparatus. (Drawing includes non-English language text)    Reformer (1)    Reforming catalyst (1a)    Evaporator (3)    Controller (10)    Source gas supply device (13)    Hydrogen generating apparatus (100A)