To provide an auxiliary machinery battery charger capable of suppressing power consumption of a main battery in charging an auxiliary machinery battery installed on a vehicle in addition to the main battery.

An auxiliary machinery battery charger 1 includes: a main battery 41 with modules 5-1 to 5-n connected in series to one another; an external generator 2; a MOSFET 13 provided between the external generator 2 and an auxiliary machinery battery 43; an operational amplifier 12 in which an output voltage of a module 7-n is input into a plus input terminal, a voltage applied to the MOSFET 13 is input into a minus input terminal and an output terminal is connected to a gate terminal of the MOSFET 13; a switch 14 provided between the external generator 2 and the MOSFET 13; and an ECU 5 which turns on the switch 14 to pass an electric current through the auxiliary machinery battery 43 via the MOSFET 13 from the external generator 2 in charging the auxiliary machinery battery 43.