A surgical device, an outer tube, an endoscope, and a treatment tool that have a compact configuration and achieve high durability are provided. The outer tube includes a slider in an outer tube body. When the endoscope and the treatment tool are inserted into the outer tube, and are held by an endoscope holding part and a treatment tool holding part each being provided in the slider, and are combined into a single unit. Consequently, when the treatment tool is moved in the axial direction, the endoscope is moved in the axial direction in interlock with the movement. Because the outer tube is configured such that the endoscope exit port has an inner diameter that is smaller than the inner diameter of the elastic member provided to hold the endoscope at the endoscope holding part, the strength of the endoscope can be secured while preventing interference with the treatment tool.
