The invention provides an in-vehicle engine start control apparatus which can be rapidly restarted even when rotation of the engine is slowed down. An in-vehicle engine start control apparatus (30A) stops a fuel injection instruction (INJ) after rotationally driving a DC electric motor (41a) preliminarily by issuing a rotational driving instruction (Rc) when an automatic stop condition is satisfied, and subsequently restarts the in-vehicle engine by issuing a push control instruction (Sc) to a pinion gear immediately before a circumferential speed of a ring gear (11) decelerating by inertia is synchronized with a circumferential speed of the pinion gear (42a) rotationally driven preliminarily to rotate and by issuing the rotational driving instruction (Rc) and the fuel injection instruction (INJ) again because a restart request is already issued or is issued with a delay when the push driving is completed.