A master-slave system comprising: an endoscope that includes an observation optical system that photographs a subject and a treatment portion that protrudes from a surface on which the observation optical system is provided and that is at least partially photographed by the observation optical system together with the subject; a master device that includes a handle that is supported so as to be capable of rotating about a predetermined axis and a treatment manipulation portion that is provided on the handle; a control portion that associates rotation of the handle of the master device with rotation of the endoscope about an axis intersecting the surface and that associates manipulation of the treatment manipulation portion with movement of the treatment portion; an image processing portion that generates a corrected image obtained by rotating an image acquired by the observation optical system by a rotation angle of the endoscope about the axis intersecting the surface; and a monitor that displays the corrected image generated by the image processing portion, wherein, when the handle is rotated up to a set angle, the control portion is configured to switch from the corrected image on the monitor to a re-corrected image obtained by additionally rotating the corrected image by a predetermined correction angle from which the absolute value of the rotation angle of the handle can be subtracted, release the association between rotation of the handle of the master device and rotation of the endoscope about the axis intersecting the surface, and resume the association when the handle is moved by an angle whose difference from the correction angle is equal to or lower than a predetermined threshold. an endoscope that includes an observation optical system that photographs a subject and a treatment portion that protrudes from a surface on which the observation optical system is provided and that is at least partially photographed by the observation optical system together with the subject; a master device that includes a handle that is supported so as to be capable of rotating about a predetermined axis and a treatment manipulation portion that is provided on the handle; a control portion that associates rotation of the handle of the master device with rotation of the endoscope about an axis intersecting the surface and that associates manipulation of the treatment manipulation portion with movement of the treatment portion; an image processing portion that generates a corrected image obtained by rotating an image acquired by the observation optical system by a rotation angle of the endoscope about the axis intersecting the surface; and a monitor that displays the corrected image generated by the image processing portion, wherein, when the handle is rotated up to a set angle, the control portion is configured to switch from the corrected image on the monitor to a re-corrected image obtained by additionally rotating the corrected image by a predetermined correction angle from which the absolute value of the rotation angle of the handle can be subtracted, release the association between rotation of the handle of the master device and rotation of the endoscope about the axis intersecting the surface, and resume the association when the handle is moved by an angle whose difference from the correction angle is equal to or lower than a predetermined threshold.