To suppress the mechanical resonance component of an actuator system worsening seek noise and stability.

In a head positioning control system in a magnetic disk device, a controller (C) 201 receives a target position 211 and information on the head position 210 calculated based on the positional information extracted from servo data read from a disk by a head, and determines a manipulated variable U for making the head move or follow up to the target position 211. A manipulated variable output part 202 branches this manipulated variable U into two systems, and independently adjusts U3, U4 of the manipulated variable U of each system by a gain-phase varying parts 207, 208 to manipulated variables U3, U4 and sums them up by an adder 209, and thereby obtains a manipulated variable U5 of which the gain is suppressed in a frequency band determined by the gain characteristics and phase characteristics of U3, U4, to input the manipulated variable U5 to a control object (P) 200.