To obtain a reflection image signal corresponding to a multiplication factor by calculating the multiplication factor of a proper electric charge multiplication factor type imaging element 106 according to a distance between the distal end of an endoscope insertion part 100 and an observation part 10 through the use of a simple calculation method, in an endoscope with which a reflection image reflected against the observation part by irradiating a light is picked up, and the reflection image signal based on the reflection image is obtained.

A reference image is acquired, which correctly reflects the distance between the observation part 10 and the distal end of the endoscope insertion part 100, so as to receive little influence from absorption in the observation part 10 and the condition of the front surface of the part 10 by irradiating a reference light (infrared light) to the observation part 100. The multiplication factor for the reflection image signal is calculated based on the reference image signal on the basis of the reference image. Thus, the reflection image signal multiplied by the multiplication factor is obtained.