An overtube used in an endoscopic surgical device including an endoscope having an endoscope insertion part to be inserted into a body cavity, a medical instrument having a medical instrument insertion part to be inserted into the body cavity, and the overtube that guides the endoscope insertion part and the medical instrument insertion part into the body cavity, the overtube comprising: an overtube body that passes through a body wall and is inserted into the body cavity; a first insertion passage that is provided inside the overtube body and allows the endoscope insertion part to be inserted therethrough so as to be movable forward and backward; a second insertion passage that is provided inside the overtube body and allows the medical instrument insertion part to be inserted therethrough so as to be movable forward and backward; and an interlocking member that includes a first coupling part coupled to the endoscope insertion part inserted through the first insertion passage and a second coupling part coupled to the medical instrument insertion part inserted through the second insertion passage, the interlocking member being arranged inside the overtube body so as to be movable forward and backward, and wherein the interlocking member has a dead zone where forward and backward movement of either the endoscope insertion part or the medical instrument insertion part does not interlock with the movement of the other and a sensing zone where the forward and backward movement of either the endoscope insertion part or the medical instrument insertion part interlocks with the movement of the other, characterized in that the first insertion passage is provided with a guide part (384, 390) that guides a distal end of the endoscope insertion part in a direction away from a distal end of the medical instrument insertion part when the endoscope insertion part is moved toward a distal end side thereof. an overtube body that passes through a body wall and is inserted into the body cavity; a first insertion passage that is provided inside the overtube body and allows the endoscope insertion part to be inserted therethrough so as to be movable forward and backward; a second insertion passage that is provided inside the overtube body and allows the medical instrument insertion part to be inserted therethrough so as to be movable forward and backward; and an interlocking member that includes a first coupling part coupled to the endoscope insertion part inserted through the first insertion passage and a second coupling part coupled to the medical instrument insertion part inserted through the second insertion passage, the interlocking member being arranged inside the overtube body so as to be movable forward and backward, and wherein the interlocking member has a dead zone where forward and backward movement of either the endoscope insertion part or the medical instrument insertion part does not interlock with the movement of the other and a sensing zone where the forward and backward movement of either the endoscope insertion part or the medical instrument insertion part interlocks with the movement of the other, characterized in that the first insertion passage is provided with a guide part (384, 390) that guides a distal end of the endoscope insertion part in a direction away from a distal end of the medical instrument insertion part when the endoscope insertion part is moved toward a distal end side thereof.