An endoscopic treatment device comprising: an insertion unit configured to be insertable into a channel of an endoscope; a manipulation unit provided at a proximal end portion of the insertion unit; and a manipulation unit attachment mechanism provided at the manipulation unit, and configured to removably attach the manipulation unit to an insertion port provided at a proximal end portion of the channel of the endoscope when the insertion unit is inserted into the channel of the endoscope, wherein the manipulation unit attachment mechanism has: a connection slider configured to be engaged with the insertion port by sliding in a first direction intersecting an axis of the insertion unit in a state in which the manipulation unit is attached to the insertion port and be disengaged from the insertion port by sliding in a second direction intersecting the axis of the insertion unit; characterized in that the manipulation unit attachment mechanism further has an insertion-port abutment part configured to be movable relative to the manipulation unit in a direction of the axis of the insertion unit and move toward a proximal end of the insertion unit in the direction of the axis of the insertion unit by coming into contact with the insertion port in the state in which the manipulation unit is attached to the insertion port; and a slider abutment part that is provided at the insertion-port abutment part and that is configured to come into contact with the connection slider to prevent sliding of the connection slider in the first direction when the insertion-port abutment part is located at a distal end side of the insertion unit in the direction of the axis of the insertion unit, and does not lock the connection slider to permit the sliding of the connection slider in the first direction when the insertion-port abutment part is moved toward a proximal end side of the insertion unit in the direction of the axis of the insertion unit. an insertion unit configured to be insertable into a channel of an endoscope; a manipulation unit provided at a proximal end portion of the insertion unit; and a manipulation unit attachment mechanism provided at the manipulation unit, and configured to removably attach the manipulation unit to an insertion port provided at a proximal end portion of the channel of the endoscope when the insertion unit is inserted into the channel of the endoscope, wherein the manipulation unit attachment mechanism has: a connection slider configured to be engaged with the insertion port by sliding in a first direction intersecting an axis of the insertion unit in a state in which the manipulation unit is attached to the insertion port and be disengaged from the insertion port by sliding in a second direction intersecting the axis of the insertion unit; characterized in that the manipulation unit attachment mechanism further has an insertion-port abutment part configured to be movable relative to the manipulation unit in a direction of the axis of the insertion unit and move toward a proximal end of the insertion unit in the direction of the axis of the insertion unit by coming into contact with the insertion port in the state in which the manipulation unit is attached to the insertion port; and a slider abutment part that is provided at the insertion-port abutment part and that is configured to come into contact with the connection slider to prevent sliding of the connection slider in the first direction when the insertion-port abutment part is located at a distal end side of the insertion unit in the direction of the axis of the insertion unit, and does not lock the connection slider to permit the sliding of the connection slider in the first direction when the insertion-port abutment part is moved toward a proximal end side of the insertion unit in the direction of the axis of the insertion unit. a connection slider configured to be engaged with the insertion port by sliding in a first direction intersecting an axis of the insertion unit in a state in which the manipulation unit is attached to the insertion port and be disengaged from the insertion port by sliding in a second direction intersecting the axis of the insertion unit; characterized in that the manipulation unit attachment mechanism further has an insertion-port abutment part configured to be movable relative to the manipulation unit in a direction of the axis of the insertion unit and move toward a proximal end of the insertion unit in the direction of the axis of the insertion unit by coming into contact with the insertion port in the state in which the manipulation unit is attached to the insertion port; and a slider abutment part that is provided at the insertion-port abutment part and that is configured to come into contact with the connection slider to prevent sliding of the connection slider in the first direction when the insertion-port abutment part is located at a distal end side of the insertion unit in the direction of the axis of the insertion unit, and does not lock the connection slider to permit the sliding of the connection slider in the first direction when the insertion-port abutment part is moved toward a proximal end side of the insertion unit in the direction of the axis of the insertion unit.