of the invention a plurality of balls 32 are rollably disposed between an outer ring 112 mounted to a bearing box 102 and an inner ring 212 mounted to a shaft 20 the inner ring 212 of the ball bearing is made of a material having a linear expansion coefficient u03b1 ji smaller than a linear expansion coefficient u03b1 s of the shaft 20 one end surface 212a of the inner ring 212 is an inclined surface which is slanted at an angle u03b8 1i extending inwardly with respect to the bearing center line 0 0 and the other end surface 212b is an inclined surface slanted at an angle u03b8 2i extending outwardly with respect to the bearing center line 0 0 both end surfaces 212a and 212b of the inner ring 212 are sandwiched between confronting surfaces of distance pieces 22 and 23 which are fixed to the shaft 20 with an interference fit the one distance piece 22 abuts against a shoulder 20b of the shaft 20 and the other distance piece 23 is secured by an inner ring clamp 26 fixed to an annular groove 20c these distance pieces 22 and 23 are made of a.