A holding mechanism for holding an optical element includes a rear side block that includes a rear side holding engagement part that is engageable with a rear side engagement part provided on a rear side of an optical element, a front side block that is coupled with the rear side block and includes a front side holding engagement part that is engageable with a front engagement part provided on a front side of the optical element or the holding member, wherein one of the rear side engagement part and the rear side holding engagement part has a groove and the other has a spherical part that has an at least partially spherical shape engageable with the groove, and wherein one of the front side engagement part and the front side holding engagement part has another groove, and the other has another spherical part that has an at least partially spherical shape engageable with the other groove.