A power source system device includes a primary power source circuit block 2 that generates a voltage (Mini+B−DropV) which is further dropped from the lowest voltage of the onboard battery that supplies power to electric and electronic parts of a vehicle, the lowest voltage occurring at a start of the engine of the vehicle; a secondary power source circuit block 3 that generates a power source voltage Vcc by boosting the voltage (Mini+B−DropV) generated by the primary power source circuit block 2, supplies the power source voltage Vcc to an integrated circuit including a control block that executes a general-purpose OS, and starts the integrated circuit; and an ACC check control circuit 4 that controls the start and stop of the operation of the primary power source circuit block 2 and secondary power source circuit block 3.