To provide an apparatus and method for controlling the rotation speed of an engine, determining the no-load state of a working machine independently of the working machine and automatically switching to idling operation.

The apparatus of controlling rotation speed of the engine includes: a storage part 8 in which a throttle opening degree threshold value corresponding to a target rotation speed of the engine 20 is stored; a first determination means reading out the throttle opening degree threshold value corresponding to the target rotation speed from the storage part 8 and determining whether or not a current throttle opening degree is equal to or smaller than the throttle opening degree threshold value; a target rotation speed determination part 16 setting the rotation speed for reducing the target rotation speed to a low idle speed when the first determination means determines that the current throttle opening degree is equal to or smaller than the throttle opening degree threshold value corresponding to the target rotation speed; and an opening degree adjustment part 14 adjusting the opening degree of a throttle valve 10 based on the target rotation speed.