A wave generator of a strain wave gearing uses a plurality of solid displacement elements to flex a flexible externally toothed gear into a non-circular shape and partially mesh the externally toothed with a rigid internally toothed gear. The solid displacement elements are obtained by curving a monomorphic or bimorphic type rectangular plate-shaped element into an arc shape and can be deformed from an arc shape to a semispherical shape by energization, the semicircular shape having a smaller radius than the arc radius of the arc shape. A wave generator can be realized, which can bear a large load torque by utilizing changes in rigidity, deformation amount, and deformation force associated with the deformation of the solid displacement elements.