NOVELTY - Gas leakage detection of fuel cell power generating device involves enclosing and sealing raw fuel gas to fuel reformer (20) after temperature of catalyst layer of reactor becomes lower than decomposition temperature of raw fuel gas, sequentially detecting temperature and pressure of sealed space, calculating pressure reduction rate based on pressure sensing value, and detecting presence or absence of airtight leak by comparison with computed pressure reduction rate based on change of volume of sealed space and temperature detected value.    USE - Gas leakage detection of fuel cell power generating device.    ADVANTAGE - The method detects presence or absence of airtight leak with sufficient precision in a short period of time.    DETAILED DESCRIPTION - Gas leakage detection of fuel cell power generating device involves enclosing and sealing raw fuel gas to fuel reformer (20) after temperature of catalyst layer of reactor becomes lower than decomposition temperature of raw fuel gas during stop of fuel cell power generating device, sequentially detecting temperature and pressure of sealed space, calculating pressure reduction rate based on pressure sensing value, and detecting presence or absence of airtight leak by comparison with computed pressure reduction rate based on change of volume of sealed space and temperature detected value. The fuel cell power generating device is provided with fuel cell main body (10) linked to fuel reformer equipped with modifier as a reactor that carries out steam reforming of raw fuel and generates reformed gas. INDEPENDENT CLAIMS are included for:    (1) a fuel cell power generating device; and    (2) a gas leakage detection apparatus of fuel cell power generating device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of the fuel cell power generating device. (Drawing includes non-English language text).    Fuel cell main body (10)    Electrolyte (11)    Anode electrode (12)    Cathode electrode (13)    Reformer (20)