A surgical device, an outer tube, an endoscope and a treatment tool that have a simple configuration and excellent operability 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, the endoscope and treatment tool 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. The outer tube is configured such that the inner diameter of the opening type hermetic valve part for the endoscope that is provided at a valve member is smaller than the inner diameter of the elastic member that is provided for causing the endoscope holding part to hold the endoscope.