The invention provides a speed controller which detects the period of an AC signal containing information of the rotational speed of a motor and controls the rotational speed based on the detected period value and a reference speed value. More particularly, the speed controller computes the amount of the deviation of the AC signal based on continuously detected period values and the reference speed value, and then stores the computed deviation amount in a memory as a correction value. Using the correction value stored in the memory, the speed controller corrects the deviation of the period, and thus, the speed of the rotation of the motor can be controlled with extremely high precision.