A wave generator (5) for a flat-type strain wave gearing (1) has a first elliptical outer circumferential surface (6S) of a first wave generator portion (5S) defined by a first elliptical curve (C1), and a second elliptical outer circumferential surface (6D) of a second wave generator section (5D) defined by a second elliptical curve (C2). Compared to using a wave generator for which the contour is defined by a single elliptical curve, differences between the ball load distribution of the first wave generator portion (5S) and the ball load distribution of the second wave generator portion (5D) can be suppressed, both ball load distributions can be equalized, and the equivalent ball load can be reduced; therefore, an increase in the lifespan of the wave generator bearing can be achieved.