A control unit 40 acquires acceleration α sensed by an acceleration sensor 31, and if the control unit 40 determines that acceleration α does not equal zero, the control unit 40 again acquires acceleration α. In the event that the control unit 40 decides that acceleration α equals zero, the control unit 40 acquires the weight M sensed by the weight sensor 30. The control unit 40 continues sampling of weight M until a predetermined sampling period has elapsed. Once the sampling period has elapsed, the control unit 40 calculates the average of weight M obtained through sampling, and uses the calculated average and a map to determine hydrogen amount.