In a hybrid circuit including a high-voltage circuit and a low-voltage circuit having different power supply voltages from each other, a voltmeter serving as a high-voltage circuit, a high-voltage connector for connecting the voltmeter and the lithium ion battery, the control unit which is a part of the low-voltage circuit and performs switching control to turn on/off internal switches of the voltmeter are mounted on a hybrid integrated circuit. Then, the hybrid integrated circuit is stacked on top of a hybrid integrated circuit mounting area of a low-voltage substrate on which the rest of the low-voltage circuit is mounted.