According to one embodiment, a magnetic disk device comprises a magnetic head including a main pole, an auxiliary magnetic pole, side shields disposed on both sides of the main pole in a track width direction with a side gap therebetween, a high frequency oscillation element disposed in the write gap between the main pole and the auxiliary magnetic pole, and a magnetic flux control element disposed in the side gap between the main pole and the side shield to control oscillation frequency of the high frequency oscillation element, an oscillation element controller configured to control bias current supplied to the high frequency oscillation element, and a magnetic flux control element controller configured to control bias current supplied to the magnetic flux control element.