To improve a control circuit for a converter head actuator in a disk driver.

Plural forward position error values PES(k) are obtained from position error signals detected in response to disturbance and outputted from a plant 64. The error values are determined covering plural time segments to be gradually increased in the forward direction. When the prescribed number of forward position error values is obtained, these error values are weighted and the weighted error values are added to the plant 64 as inputs in the reverse order of the time order obtaining these error values. In response to the inputs, plural backward propagation error values B(k) are generated from a node 60 built in a compen-sator 62 constituted as a direct form II filter function. Then these plural backward propagtion error values are used in relation with output values C (k) from the compenator 62 and the forward position error values PES (k) to calculate respective compensator gain stage values, reducing the forward position error values.