Provided is a light detecting apparatus comprising a light emitting section that emits light; a light detecting section that detects light from an observation target irradiated with the light emitted by the light emitting section; a mount section attached to a test subject that includes the observation target; and a holding section that holds the light emitting section and the light detecting section and is detachably attached to the mount section. The holding section holds the light emitting section and the light detecting section in a manner to secure a relative positional relationship between the light emitting section and the light detecting section, and a relative positional relationship between the holding section and the mount section is determined by attaching the holding section to the mount section.