An engine start device for an automatic idling stop system, smoothly and quickly causing a pinion gear and a ring gear to mesh with each other when the engine is running under the inertia. An engine start device for an automatic idling stop system is provided with: a crank angle sensor (12) for detecting the crank angle of the engine; a ring gear (11) connected to the crankshaft of the engine andtransmitting the rotation of the engine; a ring gear rotational speed detection means (13, 10) for detecting the rotational speed of the ring gear (11); a starter motor (17) for starting the engine; a pinion gear (14) for transmitting the rotation of the starter motor to the ring gear; a pinion gear push-out means (15, 16) for pushing out the pinion gear to cause the pinion gear and the ring gearto mesh with each other; and a pinion gear push-out control means (13, 10) which, when the ring gear rotational speed detected by the ring gear rotational speed detection means is below a threshold value which is determined by at least one of the crank angle, the gear range, and a restart condition, drives the pinion gear push-out means to cause the pinion gear and the ring gear to mesh with eachother.