For a balance adjustment due to a reduction in size and weight of an actuator, an arm portion, an axis attaching portion, and a coil supporting portion are arranged so that the centers of gravity on the coil side are located on a straight line connecting a rotational center and the center of gravity on the head side. Further, the improvement of an assembling efficiency of a VCM, an increase in torque, a one-touch fixing of an FPC connecting band, a noise prevention of a read pattern, and an insulation between the base and cover due to a bias of an MR read head are accomplished. A sector mark is detected by comparing a peak detection pulse and its reference polarity signal. A redundancy is provided to a reference detection polarity signal so that a read signal can be detected even if any one of the read signals of the sector marks is dropped out. A gray code detecting section for recognizing a cylinder address also detects on the basis of a peak detection pulse and a polarity signal due to the reading of a gray code. A memory space to store a program is divided into an ROM space of a small electric power consumption and an RAM space of a large electric power consumption and stores each peculiar program. When the CPU doesn't execute a program in the RAM space, the execution of an idle program in the ROM space is allocated, thereby reducing an electric power consumption in an idle state by using an ROM.