This invention provides a battery pack for driving a compact electric motor of a compact engine starting device mounted on a working machine, comprising lithium-based secondary battery comprising two cells, the entire discharge capacity of the cells being 500 to 2000 mAh, charging voltage per one cell being 3.0 to 4.2 V, various electronic circuits like driving and battery protection circuits of an electric motor, maximum discharge current of the battery having high rate of 10 to 60 A, the protection circuit including an overdischarge inhibit circuit (A) interposed in the electric motor driving circuit and automatically stopping the driving operation of the electric motor after time required for starting the engine elapsed, the volume of the battery pack being as extremely low as 2.5.times.10.sup.4 to 1.0.times.10.sup.5 mm.sup.3, the tire required until the engine is started from the actuation of the electric motor being set to an interval as short as 5 to 15 seconds, since high current can flow.