The depth of field is extended by using a simple structure. Provided is an objective optical system including an aperture stop (S) that is disposed at an intermediate position on an optical axis (O) and that has an opening (A) that allows incident light from an object to pass therethrough, wherein the aperture stop (S) includes a light-blocking portion (B) that blocks the incident light at a portion aligned with the optical axis (O). In addition, an imaging device equipped with the above objective optical system and an imaging element that acquires an optical image of an object, formed by the objective optical system, is also provided.
