A control unit is provided with an excitation current output section for outputting an excitation current to a stepping motor, a first level judging section for judging a drive stop time level indicating the duration of a time during which the stepping motor is not driven, a storage storing a plurality of starting torque values required to start driving the stepping motor and a plurality of drive stop time levels while relating them to each other, and a controller for obtaining the starting torque value corresponding to the drive stop time level judged by the first level judging section by referring to the storage, calculating an excitation current value which gives the obtained starting torque value and outputting an excitation current having the calculated excitation current value.