The present invention discloses a surgical device (1) for transanally accessing the rectum of a patient comprises a circular anal dilator (2) having a cylindrical ring wall (6) adapted to be transanally inserted into the rectum in order to provide a transanal access opening therethrough and a connecting portion (8) arranged at a proximal end of the ring wall (6) and adapted to hold the ring wall (6) inside the rectum, an anoscope (3) comprising a first retaining wall (13) defining at least partially a working channel (14) and a window aperture (15) providing an access area (16) to the surrounding tissue from inside the working channel (14), said anoscope (3) being received by the ring wall (6) of the anal dilator (2) and having a length such that it protrudes distally from the ring wall (6), a shield member (4) comprising a second retaining wall (26) which can be arranged in a manner to partially overlap the window aperture (15) of the anoscope (3) in order to vary the tissue access area (16) thereof.