High-precision observation is made possible while allowing contact between a distal end of an objective lens and an optical element without damaging the distal end of the objective lens and the optical element even when attaching to and detaching from the distal end of the objective lens. Provided is an objective lens adapter including a fixed member that is fixed to a lens tube of an objective lens, a distal-end member including an optical element that is made to be placed in contact with the distal-end surface of the objective lens, and an elastic member that is disposed between the distal-end member and the fixed member and that urges the optical element in a direction that causes the optical element to contact the distal-end surface of the objective lens.