An outer tube to be used in a surgical apparatus for an endoscope including a first medical instrument having a first insertion part to be inserted into a body cavity, a second medical instrument having a second insertion part to be inserted into the body cavity, and an outer tube that passes through a body wall, is inserted into the body cavity, and guides the first insertion part and the second insertion part into the body cavity, the outer tube comprising: an outer tube body (320, 340, 360) 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 (320, 340, 360); a first base end opening and a second base end opening provided at the base end of the outer tube body (320, 340, 360); a first insertion passage that is provided along the longitudinal axis of the outer tube body (320, 340, 360), allows the first distal end opening and the first base end opening to communicate with each other, and has the first insertion part to be inserted therethrough so as to be movable forward and backward; a second insertion passage that is provided along the longitudinal axis of the outer tube body (320, 340, 360), allows the second distal end opening and the second base end opening to communicate with each other, and has the second insertion part to be inserted therethrough so as to be movable forward and backward; an interlocking member that has a first coupling part to be coupled to the first insertion part inserted through the first insertion passage and a second coupling part to be coupled to the second insertion part inserted through the second insertion passage, and is movable forward and backward inside the outer tube body (320, 340, 360); characterized in that a fixed member that is provided in at least one couple part of the first coupling part or the second coupling part, has an inner peripheral surface contacting an entire outer peripheral surface, in a circumferential direction, of an insertion part to be coupled to the at least one coupling part, is made of a foamed elastic body having a number of pores, and makes the pores hold an oil component. an outer tube body (320, 340, 360) 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 (320, 340, 360); a first base end opening and a second base end opening provided at the base end of the outer tube body (320, 340, 360); a first insertion passage that is provided along the longitudinal axis of the outer tube body (320, 340, 360), allows the first distal end opening and the first base end opening to communicate with each other, and has the first insertion part to be inserted therethrough so as to be movable forward and backward; a second insertion passage that is provided along the longitudinal axis of the outer tube body (320, 340, 360), allows the second distal end opening and the second base end opening to communicate with each other, and has the second insertion part to be inserted therethrough so as to be movable forward and backward; an interlocking member that has a first coupling part to be coupled to the first insertion part inserted through the first insertion passage and a second coupling part to be coupled to the second insertion part inserted through the second insertion passage, and is movable forward and backward inside the outer tube body (320, 340, 360); characterized in that a fixed member that is provided in at least one couple part of the first coupling part or the second coupling part, has an inner peripheral surface contacting an entire outer peripheral surface, in a circumferential direction, of an insertion part to be coupled to the at least one coupling part, is made of a foamed elastic body having a number of pores, and makes the pores hold an oil component.