To provide a method for controlling rotation speed of a motor on a construction machine increasing and decreasing rotation speed of the motor based on intention of an operator.

A foot switch 52 which the operator turns on and off by a foot is provided at an operator's seat, and a rotation speed adjusting dial 54 for the motor 30, an operation mode selecting switch 56 below the same, and a switch 58 making on and off of the foot switch effective below the operation mode selecting switch are provided at the operator's seat. The motor is a diesel engine, and is set to sweet spot rotation speed Ns thereof. When the operator judges that it is better to increase motor rotation speed and turns the foot switch on at time t1, motor rotation speed smoothly increase to predetermined rated rotation speed Nr following a pattern A. If operation judged by the operator is completed after time t2, motor rotation speed smoothly reduced following a pattern B and returns to rotation speed Ns when the foot switch is turned off at time t3.