A light detecting module that detects interference light that has exited an exit end surface of an optical fiber in an OCT instrument includes: a ball lens including an incident surface entered by the interference light that has exited the exit end surface, and an exit surface exited by the interference light that has entered the incident surface; and a photodiode including a detecting surface entered by the interference light that has exited the exit surface. The interference light obliquely enters the incident surface with respect to a perpendicular line at an incident position of the interference light. The interference light obliquely exits the exit surface with respect to a perpendicular line at an exit position of the interference light. The interference light obliquely enters the detecting surface with respect to a perpendicular line at an incident position of the interference light.