According to the present invention, there is provided a fuel reserving device for reserving fuel therein comprising a wall for dividing an interior of the device to a fuel chamber and an air chamber, the wall being deformable according to the amount of the fuel in the fuel chamber, a discharge passage which is open to a space formed above the surface of the fuel in the fuel chamber and a shut off valve for normally shutting the discharge passage off. Gas is discharged from the space via the discharge passage when the shut off valve is open. The shut off valve is opened and the gas is discharged from the space when the amount of the gas is larger than a predetermined amount. On the other hand, the shut off valve is closed and the discharging operation of the gas is stopped when the amount of the gas is smaller than the predetermined amount.