To provide a battery capacity determination device to determine the battery capacity during cranking which enhances the determination accuracy by acquiring information necessary and sufficient for determination of the battery capacity during the cranking time.

The battery capacity determination device comprises: an engine control device 15 to control the operation of an engine 16; a battery capacity determination means which is started by the start of the cranking to determine the capacity of a battery 12 during the cranking; a measurement means to measure the cranking time; and a start prohibiting means to prohibit the start of the engine 16 before the cranking time elapses the predetermined battery capacity determination time.