A side viewing endoscope system includes a mother endoscope (1) of a side viewing type, having an instrument raising chamber (14) including an instrument raiser member (15) for raising up or sinking down of a treatment instrument, and a biopsy channel (13) provided in an axial direction of the insertion section (3) on the endoscope and in communication with the instrument raising chamber. A daughter endoscope (100) of the front viewing type, with a slender insertion section (102), can be inserted into the biopsy channel of the mother endoscope to a window (52, 53) formed in the rigid distal end portion (3a) of the mother endoscope. Means can be provided to ensure that the instrument raiser member (15) does not interfere with the slender insertion section (102), and to ensure that the slender insertion section (102) does not project from the rigid distal end portion (3a).
