An endoscopic surgical apparatus comprising: an endoscope having an observation window at a distal end thereof; a treatment tool having a treatment part at a distal end thereof; and an outer tube that passes through a body wall, is inserted into a body cavity, and guides the endoscope and the treatment tool into the body cavity, wherein the outer tube includes an outer tube body having a distal end, a base end, and a longitudinal axis, a first distal end opening and a second distal end opening provided at the distal end of the outer tube body, a first base end opening and a second base end opening provided at the base end of the outer tube body, an endoscope insertion passage that is provided along the longitudinal axis of the outer tube body, allows the first distal end opening and the first base end opening to communicate with each other therethrough, and allows the endoscope to be inserted therethrough so as to be movable forward and backward, a treatment tool insertion passage that is provided along the longitudinal axis of the outer tube body, allows the second distal end opening and the second base end opening to communicate with each other therethrough, and allows the treatment tool to be inserted therethrough so as to be movable forward and backward, an interlocking member that has an endoscope-coupling part coupled to the endoscope inserted through the endoscope insertion passage, and a treatment tool-coupling part coupled to the treatment tool inserted through the treatment tool insertion passage and is movable forward and backward inside the outer tube body, a first stopper that is provided inside the outer tube body and restricts movement of the interlocking member to the distal end side of the outer tube body, and a second stopper that is provided inside the outer tube body and restricts movement of the interlocking member to the base end side of the outer tube body, and characterised in that the endoscope has a positioning part at a position apart by at least Lt+L1 from a distal end of the endoscope toward a base end side thereof in which a movement distance of the interlocking member when the interlocking member moves from a first position where movement thereof to the distal end side of the outer tube body is restricted by the first stopper to a second position where movement thereof to the base end side of the outer tube body is restricted by the second stopper is defined as L1 and a length of the outer tube body in a longitudinal axis direction is defined as Lt, such that when the endoscope is mounted on the outer tube by inserting the endoscope through the endoscope insertion passage of the outer tube the positioning part the insertion part of the endoscope is matched with the position of the base end of the outer tube. an endoscope having an observation window at a distal end thereof; a treatment tool having a treatment part at a distal end thereof; and an outer tube that passes through a body wall, is inserted into a body cavity, and guides the endoscope and the treatment tool into the body cavity, wherein the outer tube includes an outer tube body having a distal end, a base end, and a longitudinal axis, a first distal end opening and a second distal end opening provided at the distal end of the outer tube body, a first base end opening and a second base end opening provided at the base end of the outer tube body, an endoscope insertion passage that is provided along the longitudinal axis of the outer tube body, allows the first distal end opening and the first base end opening to communicate with each other therethrough, and allows the endoscope to be inserted therethrough so as to be movable forward and backward, a treatment tool insertion passage that is provided along the longitudinal axis of the outer tube body, allows the second distal end opening and the second base end opening to communicate with each other therethrough, and allows the treatment tool to be inserted therethrough so as to be movable forward and backward, an interlocking member that has an endoscope-coupling part coupled to the endoscope inserted through the endoscope insertion passage, and a treatment tool-coupling part coupled to the treatment tool inserted through the treatment tool insertion passage and is movable forward and backward inside the outer tube body, a first stopper that is provided inside the outer tube body and restricts movement of the interlocking member to the distal end side of the outer tube body, and a second stopper that is provided inside the outer tube body and restricts movement of the interlocking member to the base end side of the outer tube body, and characterised in that the endoscope has a positioning part at a position apart by at least Lt+L1 from a distal end of the endoscope toward a base end side thereof in which a movement distance of the interlocking member when the interlocking member moves from a first position where movement thereof to the distal end side of the outer tube body is restricted by the first stopper to a second position where movement thereof to the base end side of the outer tube body is restricted by the second stopper is defined as L1 and a length of the outer tube body in a longitudinal axis direction is defined as Lt, such that when the endoscope is mounted on the outer tube by inserting the endoscope through the endoscope insertion passage of the outer tube the positioning part the insertion part of the endoscope is matched with the position of the base end of the outer tube.