NOVELTY - The hydrogen producing apparatus (100) has a raw material supply unit (9), a water supply unit (8), a modifier (1), an oxidation gas supply unit (5), carbon monoxide selective oxidation device (3), temperature control-unit (15) and control apparatus (7). The catalyst is maintained from an operating temperature to high temperature, beyond predetermined processing time, and performs high temperature processing.    USE - For producing hydrogen used as fuel for fuel cell system (claimed).    ADVANTAGE - The hydrogen producing apparatus quickly recovers the decreasing capability of carbon monoxide concentration of carbon monoxide selective oxidation device.    DETAILED DESCRIPTION - The hydrogen producing apparatus has a raw material supply unit, a water supply unit, a modifier, an oxidation gas supply unit, carbon monoxide selective oxidation device, temperature control-unit and control apparatus. The modifier modifies the raw material supplied from raw material supply unit, to reformed gas by modification reaction. The oxidation gas supply unit adds oxidation gas containing oxygen to reformed gas. The oxidation device decreases the concentration of carbon monoxide of oxidation gas-added reformed gas by carbon monoxide selective oxidation. The control unit controls the temperature of carbon monoxide selective oxidation catalyst in the carbon monoxide selective oxidation device. The control apparatus controls the supply units and generation of preformed gas in modifier, oxidation gas supply unit, and stops supply of oxidation gas. The catalyst is maintained from an operating temperature to high temperature, beyond predetermined processing time, and performs high temperature processing. INDEPENDENT CLAIMS are included for the following:    (1) fuel cell system;    (2) operating method of hydrogen producing apparatus; and    (3) operating method of fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows the systematic diagram of an outline structure of the fuel cell system. (Drawing includes non-English language text).    modifier (1)    transformer (2)    carbon monoxide selective oxidation device (3)    temperature detector (4)    air supply unit (5)    control apparatus (7)    water supply unit (8)    raw material supply unit (9)    combustor (10)    temperature control unit (15)    hydrogen producing apparatus (100)