A medical treatment endoscope includes: an insertion portion which has a lumen formed along a longitudinal axis of the insertion portion and is configured to be inserted into a body cavity, a lateral opening portion which extends in a lateral surface of the insertion portion, from a distal end surface of the insertion portion to a proximal end side of the insertion portion, an elastic member which is provided in an inner wall of the lumen at a position separated from an edge of a proximal end of the insertion portion in a direction toward a distal end side of the insertion portion, and a tubular arm which includes a bendable bending part provided at a distal end side of the tubular arm.