A main spindle device of a machine tool includes a plurality of bearings that rotatably support a main spindle of the machine tool, and that are placed inside a housing with a pressure receiving member interposed between the bearings and the housing. The pressure receiving member is capable of moving in a direction perpendicular to an axial direction of the main spindle. A pressure chamber, which a pressure medium pressing the pressure receiving member toward the bearings in the perpendicular direction is supplied to, is formed in the housing. A plurality of the pressure chambers are independently formed in the housing so as to correspond to the bearings, respectively. The main spindle device further includes a plurality of adjustment units provided independently so as to correspond the pressure chambers, respectively, and each capable of independently adjusting a pressure of the pressure medium for a corresponding one of the pressure chambers.