An apparatus for controlling a driving speed of a motor comprises a motor controller provided in the motor controlling apparatus detecting the motor driving speed in response to pulses outputted from an encoder. The motor controller adjusts a detection period for the pulses according to the motor driving speed, thereby measuring the motor driving speed. The motor controller sets the detection period so that a number of four-multiplied pulses can be a multiple of four. Although a phase difference between pulses outputted from an encoder is not constant, the motor controlling apparatus can obtain a desired response characteristic by excluding effects of a phase error between the pulses.