In a device for controlling the rotational speed of a motor according to the present invention, feedback control of the number of revolutions of the motor is carried out by a speed difference control signal and a phase difference control signal so that the motor is rotated at constant speed. The phase difference which is a basis of the phase difference control signal is found in a control portion (14) in the following manner. A first time period from the reference time point to the time when the newest detection pulse is outputted is calculated every time a predetermined number of detection pulses are counted by detection pulse counters (131, 133, 134, 136), a second time period from the reference time point to the time when a command pulse, which is assigned an ordinal number equal to the ordinal number of the newest detection pulse, corresponding to the newest detection pulse is outputted is calculated on the basis of the total number of pulses counted by the detection pulse counters in the time period from the reference time point to the time when the newest detection pulse is outputted and the period of the command pulse, and the phase difference between the newest detection pulse and the command pulse corresponding thereto is calculated on the basis of the first time period and the second time period calculated and the period of the command pulse. A phase difference control signal proportional to the phase difference calculated is determined.