To provide a fuel consumption measuring device capable of measuring fuel consumption without requiring drastic modification in an automobile side and applicable to measurement of fuel consumption of a stationary fuel cell power generating device and a conventional internal combustion engine without being limited to a fuel cell vehicle.

A supplying oxygen state measuring means 100 calculates oxygen amount supplied to a power source 10, and a discharging oxygen state measuring means 200 calculates oxygen amount discharged from the power source 10. An oxygen consumption arithmetic circuit 300 calculates oxygen consumption in the power source 10 based on the supplied and discharged oxygen amount. A fuel consumption arithmetic circuit 400 determines fuel amount consumed by the power source 10 based on the oxygen consumption.