An objective-optical-system positioning apparatus includes a positioning unit interposed between an objective optical system having a small-diameter end section and an examination optical system for examining light collected by the objective optical system; and a substantially cylindrical support unit, one end of which is secured to an organism, and which internally supports the small-diameter end section in a detachable manner. The positioning unit includes a holding part for holding the objective optical system and a moving mechanism supporting the holding part such that the holding part is freely movable in a direction intersecting with a direction of an optical axis of the objective optical system, and the other end of the support unit has a tapered inner surface whose diameter gradually increases toward the tip.