An insertion unit has a channel through which a treatment tool can be inserted. A distal end of the insertion unit has a single-focus objective optical system or a focal-point-variable objective optical system to focus the optical image on a light receiving surface of an image capturing element. At a short object distance from the distal end, an image signal with a sufficient resolution is obtained from the image capturing element. In this state, a treatment tool projected by a reduced amount from a distal end opening of the channel is captured on a light receiving surface of the image capturing element. A required resolution is also provided for a far side.