Abstract: An actuator mechanism for positioning magnetic heads includes arms that support a magnetic recording/reproducing head at one end thereof. Each of the arms is mounted about a shaft and supported for rotation. A coil support member is attached to the end of the arms opposite the head, and is made of a molded resin that integrally covers a voice coil motor used for causing the swinging movement of the arms. The molded resin of the coil support member is a synthetic resin mixed with a powered metal having a known specific gravity and thermal expansion coefficient. Thus, the weight of the synthetic resin can be adjusted to balance a portion including the magnetic heads and the arms with the coil support member in relation to the shaft.