An insertion module inserted into inside of an observed object includes a light-source light emitting module, around its distal end, configured to emitted light from a light source and a detector, provided close to the distal end, configured to detect a light quantity of incident visible light. A controller controls the light source such that a null signal is transmitted from the light-source light emitting module, the null signal being characteristic for detection of the position of the distal end and obtained by making at least light of a predetermined wavelength region have an absence of visible light. The detector performs detecting operation in a period in which the null signal is transmitted. A determination module performs determination relating to the position of the distal end, based on detection information outputted by the detector.