A wave generator for a strain wave gearing, the wave generator comprising: a rigid plug having a non-circular outer peripheral surface or a non-circular inner peripheral surface; a wave bearing capable of being flexed radially, the wave bearing being mounted to the non-circular outer peripheral surface or the non-circular inner peripheral surface and being flexed into a non-circular shape; and a gap securing member, wherein the wave bearing comprises: an inner race-side raceway surface and an outer race-side raceway surface that are flexed into a non-circular shape by the rigid plug; and a plurality of rolling elements inserted between the inner race-side raceway surface and the outer race-side raceway surface, characterized in that the gap securing member rotates integrally with the rigid plug, part of the rolling elements is in a loose state and the remaining rolling elements are in a tight state, and wherein, where one pair of adjacent rolling elements among the rolling elements are referred to as a first rolling element and a second rolling element, the first rolling element being in the loose state and the second rolling element being in the tight state, the gap securing member is a member applying a braking force to the first rolling element against an orbital motion thereof in a direction toward the second rolling element, to secure a gap between the first rolling element and the second rolling element i mmediately before the first rolling element transitions from the loose state into the tight state. a rigid plug having a non-circular outer peripheral surface or a non-circular inner peripheral surface; a wave bearing capable of being flexed radially, the wave bearing being mounted to the non-circular outer peripheral surface or the non-circular inner peripheral surface and being flexed into a non-circular shape; and a gap securing member, wherein the wave bearing comprises: an inner race-side raceway surface and an outer race-side raceway surface that are flexed into a non-circular shape by the rigid plug; and a plurality of rolling elements inserted between the inner race-side raceway surface and the outer race-side raceway surface, characterized in that the gap securing member rotates integrally with the rigid plug, part of the rolling elements is in a loose state and the remaining rolling elements are in a tight state, and an inner race-side raceway surface and an outer race-side raceway surface that are flexed into a non-circular shape by the rigid plug; and a plurality of rolling elements inserted between the inner race-side raceway surface and the outer race-side raceway surface, characterized in that the gap securing member rotates integrally with the rigid plug, part of the rolling elements is in a loose state and the remaining rolling elements are in a tight state, and wherein, where one pair of adjacent rolling elements among the rolling elements are referred to as a first rolling element and a second rolling element, the first rolling element being in the loose state and the second rolling element being in the tight state, the gap securing member is a member applying a braking force to the first rolling element against an orbital motion thereof in a direction toward the second rolling element, to secure a gap between the first rolling element and the second rolling element i mmediately before the first rolling element transitions from the loose state into the tight state. where one pair of adjacent rolling elements among the rolling elements are referred to as a first rolling element and a second rolling element, the first rolling element being in the loose state and the second rolling element being in the tight state, the gap securing member is a member applying a braking force to the first rolling element against an orbital motion thereof in a direction toward the second rolling element, to secure a gap between the first rolling element and the second rolling element i mmediately before the first rolling element transitions from the loose state into the tight state.