Provided is a surgical apparatus for an endoscope with which an operator can change, as desired, the size of an image of a site to be observed that appears in an endoscopic image obtained by an endoscope or the size of the range of the site to be observed. An outer tube that guides an endoscope 100 and a treatment tool 200 into a body cavity includes a slider 400 that is an interlocking member that moves the endoscope 100 and the treatment tool 200 forward and backward in an interlocking manner. The slider 400 has a non-sensing region where the forward and backward movement of either the endoscope 100 or the treatment tool 200 does not interlock with the movement of the other and a sensing region where the forward and backward movement of either the endoscope 100 or the treatment tool 200 interlocks with the movement of the other. An image processing unit 152 (zooming means) of a processor device 108 changes a zoom magnification factor of an endoscopic image from an endoscope 100 through electronic zooming on the basis of the operation of a foot switch 156. In the non-sensing region of the slider 400, the activation and deactivation of the zooming operation by the foot switch 156 are switched according to a relative position of a distal end of the endoscope 100 with respect to a distal end of the treatment tool 200.