A bearing device for a high speed rotary shaft is provided with a functional bearing disposed on a shaft which is supported rotatably on a housing by a pair of bearings (5), and capable of supporting rotatably the shaft, a rotation sensor for detecting the speed of revolution of the shaft and a controller for changing the functional bearing to a shaft supporting state in response to a rotation detection signal from the rotation sensor representing that the speed of revolution of the shaft is above a predetermined level. When the shaft rotates at a low speed, the functional bearing is set to a free state and the shaft is supported at two points on the housing by the pair of bearings (5) alone. When the shaft rotates at a high speed, the functional bearing is operated so that the shaft is supported at three points on the housing to prevent the occurrence of deflection of the shaft.