Error signal detection circuit in a servo control device

An error signal detection circuit in a servo control device comprises a reference signal generation circuit for selectively generating reference signals of different periods, each having a slope which changes from a reference value at a substantially constant gradient, an error detection timing generation circuit for generating timing of detection of an error signal of an object of control, an error signal detection circuit for generating an error signal by detecting the value of a reference signal generated by the reference signal generation circuit at a timing provided by the error detection timing generation circuit, and a slope varying circuit for varying the slope of the reference signal in accordance with the period of the reference signal. By changing the gradient of the slope of the reference signal in accordance with the reference period, the ratio of change of the error signal output to the error can be maintained substantially constant so that the gain of the servo loop can be maintained substantially constant and optimalization of the servo system can thereby be realized.