A trocar for thoracoscopic surgery includes a cylindrical portion with an elliptical cross section, and an outer flange and an inner flange provided on respective ones of both ends of the cylindrical portion. The cylindrical portion has two planar side wall portions, two arcuate portions connecting the side wall portions at both ends thereof, and a partitioning wall portion partitioning the interior of the cylindrical portion between both side wall portions. Both side wall portions per se and both arcuate portions per se are formed in curved fashion in such a manner that outer surfaces thereof take on a concave shape.
