A light detecting apparatus which can be arranged even in an optical path of which converting angle is relatively large, and can accurately detect light entering along a predetermined direction in an optical path. The light detecting apparatus (10) for detecting light which enters in a predetermined direction in an optical path has a light splitting element (11) having two optical surfaces (11a, 11b) positioned in the optical path, and a photoelectric detector (12) for photo detecting light which propagates inside the light splitting element and which is guided from a side face (11c) of the light splitting element. The light splitting element further has an incident angle conversion section (13) for converting a part of light which enters one optical face of the light splitting element into light entering the other optical surface at an incident angle greater than or equal to a total reflection angle.