According to the present invention, a transmission controller performs a self-diagnosis of a device when a first voltage is higher than a first threshold value, at which value a first switch is judged to be normal, a second voltage is between a second threshold value, at which value the first switch and a second switch are both judged to be normal, and a minimum voltage, at which voltage device operation can be assured, and a difference between the first voltage and the second voltage is smaller than a third threshold value, at which value the second switch is judged to be normal, and determines that low-voltage abnormality in which input voltage to the device is insufficient is caused when the device does not operate normally in the self-diagnosis.
