To provide a prime motor starting device capable of positively-limiting current, so as to prevent a starter motor from being overloaded at initial start.

This prime mover starting device is provided with a starter motor 4 for driving the prime motor, until it reaches prescribed speed at the time of the initial start, a current-limiting resistor 7 for limiting current flowing through the starter motor 4 at the beginning of the start, a set of two batteries 5, 6 which supply electric power to the starter motor 4, and a plurality of switches 11, 12 conducting connection-switching or turning on/off the resistor 7 and the set of two batteries 5, 6 in accordance with a prescribed starting sequence. The prescribed starting sequence connects two plurality of batteries 5, 6 connected in parallel and the current limiting resistor 7 in series to the starter motor 4 to start it, and after a prescribed time, connects the set of two batteries 5, 6 subjected to connection switching in series directly to the starter motor 4 to continue starting.