An endoscopic treatment device includes an insertion unit, a manipulation unit provided at a proximal end portion of the insertion unit, and a manipulation unit attachment mechanism attaching the manipulation unit to an insertion port of a channel of an endoscope. The manipulation unit attachment mechanism has: a connection slider engaged with the insertion port by sliding in a first direction intersecting an axis of the insertion unit and disengaged from the insertion port by sliding in a second direction intersecting the axis； an insertion-port abutment part moving toward a proximal end by contacting with the insertion port； and a slider abutment part preventing sliding of the connection slider in the first direction when the insertion-port abutment part is located at a distal end side and permitting the sliding of the connection slider in the first direction when the insertion-port abutment part is located at a proximal end side.
