An observation unit can be stably held in a state where the observation unit is mounted on a distal portion, and can be easily removed without interfering with a connection pipe. A mounting hole for mounting an observation unit on a distal end body which constitutes a distal portion is formed. The mounting hole is formed with a first holding portion into which the distal portion of the lens barrel is inserted, a second holding portion into which a prism holder is inserted, and a releasing portion having a space larger than the external diameter of the lens barrel between the first and second holding portions.