In a control device for a power supply circuit in which a plurality of high-voltage battery packs are provided, if any one of safety switch is off, the startup state of a high-voltage circuit is totally prevented, even when a careless mistake such as a careless power-on (ON) operation is made, the circuit is prevented from being started, and also the overall power supply is cut off by turning off (OFF) one safety switch, thereby improving the convenience of work. Voltage sensing units (9-1 to 9-n) that sense internal voltages are provided in battery packs (4-1 to 4-n) to detect a voltage change based on the operation of one or more safety switches (8-1 to 8-n) and maintain all relays (5-1 to 5-n) of the plurality of battery packs (4-1 to 4-n) provided in parallel in an open (OPEN) state.