An inverter device provided on a washing machine for controlling a permanent magnet motor being provided with a rotor magnet including a first permanent magnet and a second permanent magnet having a level of coercivity smaller than the first permanent magnet. An excitation current is produced to vary the amount of magnetism of the second permanent magnet to execute a dehydrate operation with a magnetic flux of the rotor magnet reduced and to execute an operation being specified to operate at a lower maximum rotation count compared to the dehydrate operation with the magnetic flux of the rotor magnet increased. When the amount of magnetism of the second permanent magnet is varied while rotation of the permanent magnet motor is stopped, a phase of the excitation current for varying the amount of magnetism is switched depending on a rotation stop position of the rotor.