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 so as to be integrated. When the treatment tool is moved in the axial direction, the endoscope is moved in the axial direction in interlock with the movement. In the outer tube, the opening type hermetic valve part for the endoscope in a valve member has the inner diameter larger than the inner diameter of the elastic member for causing the endoscope holding part to hold the endoscope.