A wave generator (4) of a wave gear device (1) is formed from a material in which only the minor axis-side portions (51, 52), which include the two end portions (51a, 52a) of an elliptical minor axis Lmin, has a larger coefficient of linear expansion than the other portions. When the temperature of a rigid cam plate (5) increases during high-speed rotational input, the minor axis-side portions (51, 52) thermally expand considerably more than the other portions do, and balls (10) other than several balls (10b) on a minor axis Lmin of a wave bearing (7) enter a tight state and form a locked state in which the wave bearing (7) rotates integrally with the rigid cam plate (5). Since the generation of agitation resistance of the lubricating agent due to the high-speed rolling movement of the balls (10) during high-speed rotational input can be avoided or reduced, a reduction of the torque transmission efficiency caused by the agitation resistance can be prevented or inhibited.