A remaining gas amount calculating device includes a hydrogen tank, an opening/closing valve arranged to deliver hydrogen gas from inside the hydrogen tank, a decompression valve arranged to decompress hydrogen gas delivered from the hydrogen tank, a pressure sensor arranged to measure the pressure of hydrogen gas decompressed with the decompression valve, and a power source system controller arranged to determine the remaining gas amount in the hydrogen tank by calculating the gas pressure in the hydrogen tank based on the hydrogen gas pressure measured with the pressure sensor. The relationship between pressures in a primary chamber and a secondary chamber of the decompression valve is monotonical such that when the pressure in the primary chamber increases, the pressure in the secondary chamber also increases; and when the pressure in the primary chamber decreases, the pressure in the secondary chamber also decreases. The remaining gas amount calculating device makes it possible to calculate the amount of gas remaining in a gas container without using a gas pressure measuring device that measures directly the pressure in the gas container.