An excitation circuit includes a first switch connected between a third signal line and a first signal line supplied with a first DC voltage, a second switch connected between the third signal line and a second signal line supplied with a second DC voltage higher than the first DC voltage, third to sixth switches that reverses a polarity of a voltage on the third signal line to change a direction of an excitation current, a switching control circuit that switches the first switch at intervals shorter than intervals at which an excitation polarity is switched so that the excitation current has a first target value, and a switching control circuit that turns on the second switch when the absolute value of the excitation current is smaller than a second target value less than or equal to the first target value and that turns off the second switch when the excitation current is larger than the second target value.