PURPOSE: To stabilize the number of revolutions of an engine during idling, in a fuel injection quantity control device for a diesel engine to control a fuel injection quantity through control of drive of a spill valve.

CONSTITUTION: The allowable minimum target number of revolutions of an engine is determined based on a surplus angle computed by a surplus angle computing means A7. A target number of revolutions of engine correcting means A10 is provided for correcting the target number of revolutions of an engine according to the minimum target number of revolutions of an engine when the minimum target number of revolutions of an engine is different from the target number of revolutions of an engine, computed by a number of idle revolutions control means A2, by a given value or more.