A servo controlling method of the present invention is a servo controlling method for controlling a position of a magnetic head against a magnetic tape that is accommodated in a cartridge and has at least a data track and a servo track, and the method includes the steps of: reading tape variance information from a magnetic tape cartridge (23) side, the magnetic tape cartridge (23) being composed of a cartridge (20) and a magnetic tape (21); reading a servo signal recorded in the servo track by the magnetic head and converting the servo signal into head position information; and controlling the position of the magnetic head according to an off-track value based on the tape variance information and an off-track value calculated from the head position information. With this configuration, even in the case where a recording track width is narrowed and a tape speed is increased, the magnetic head can follow a recording track accurately at a high speed.