An optical element mounting and demounting device for bringing an optical element (26) into an optical path of an optical lens system and removing it from the optical path includes a holder frame (28) for holding the optical element (26) that is supported for pivotal movement on a stationary shaft (30) so as to move the optical element (26) between a rest position and a working position, an urging member (40, 50, 60) for applying urging force against the holder frame during pivotal movement of the holder frame (28) between the rest position and the working position, the urging member being secured to a stationary member so as to invert the direction of urging force against the holder frame between the rest position and the working position, and a manually operable member elements for causing pivotal movement of the holder frame between the rest position and the working position.