An aperture stop mechanism comprising: a base plate having a circular fixed aperture hole; two aperture blades (14, 16; 114, 116; 214, 216) operative to be arranged so as to overlap each other and form a single light passage opening (50; 150; 250) with the respective edge portions (14g, 14h, 16g, 16h; 114g, 114h, 116g, 116h; 214g, 214h, 214i, 216g, 216h, 216i) of the two aperture blades (14, 16), into which light utilized in imaging is introduced, and a drive mechanism operative to drive the two aperture blades (14, 16; 114, 116; 214, 216) along a first direction opposite to each other between a predetermined maximum aperture position and a predetermined minimum aperture position to thereby change an aperture area of the single light passage opening (50; 150; 250) while light-adjusting the circular fixed aperture hole, wherein the two aperture blades (14, 16; 114, 116; 214, 216) are provided so as to form the single light passage opening (50; 150; 250), surrounded by boundaries including n straight segments, with n = 6, 8, 10, formed by the respective edge portions (14g, 14h, 16g, 16h; 114g, 114h, 116g, 116h; 214g, 214h, 214i, 216g, 216h, 216i) of the two aperture blades (14, 16; 114, 116; 214, 216), release light shielding of the circular fixed aperture hole so that the circular fixed aperture hole is fully opened at the predetermined maximum aperture position, form the single light passage opening (50; 150; 250) having an n-polygonal flat-shape at the predetermined minimum aperture position, the n-polygonal flat-shape having a width of the n-polygon in the first direction which is smaller than the width in a second direction perpendicular to the first direction, form the single light passage opening (50; 150; 250) of a regular n-polygon at a predetermined proximate position between the maximum aperture position and the minimum aperture position, and form the single light passage opening (50; 150; 250) such that a circular arc of the fixed aperture hole and of at least some of the edge portions (14g, 16g; 114g, 116g; 214g, 216g) are combined at a predetermined halfway position between the maximum aperture position and the predetermined proximate position, the edge portions (14g, 14h, 16g, 16h; 114g, 114h, 116g, 116h; 214g, 214h, 214i, 216g, 216h, 216i) forming an n-polygonal flat-shape having a width of the n-polygon in the first direction which is larger than the width in the second direction. a base plate having a circular fixed aperture hole; two aperture blades (14, 16; 114, 116; 214, 216) operative to be arranged so as to overlap each other and form a single light passage opening (50; 150; 250) with the respective edge portions (14g, 14h, 16g, 16h; 114g, 114h, 116g, 116h; 214g, 214h, 214i, 216g, 216h, 216i) of the two aperture blades (14, 16), into which light utilized in imaging is introduced, and a drive mechanism operative to drive the two aperture blades (14, 16; 114, 116; 214, 216) along a first direction opposite to each other between a predetermined maximum aperture position and a predetermined minimum aperture position to thereby change an aperture area of the single light passage opening (50; 150; 250) while light-adjusting the circular fixed aperture hole, wherein the two aperture blades (14, 16; 114, 116; 214, 216) are provided so as to form the single light passage opening (50; 150; 250), surrounded by boundaries including n straight segments, with n = 6, 8, 10, formed by the respective edge portions (14g, 14h, 16g, 16h; 114g, 114h, 116g, 116h; 214g, 214h, 214i, 216g, 216h, 216i) of the two aperture blades (14, 16; 114, 116; 214, 216), release light shielding of the circular fixed aperture hole so that the circular fixed aperture hole is fully opened at the predetermined maximum aperture position, form the single light passage opening (50; 150; 250) having an n-polygonal flat-shape at the predetermined minimum aperture position, the n-polygonal flat-shape having a width of the n-polygon in the first direction which is smaller than the width in a second direction perpendicular to the first direction, form the single light passage opening (50; 150; 250) of a regular n-polygon at a predetermined proximate position between the maximum aperture position and the minimum aperture position, and form the single light passage opening (50; 150; 250) such that a circular arc of the fixed aperture hole and of at least some of the edge portions (14g, 16g; 114g, 116g; 214g, 216g) are combined at a predetermined halfway position between the maximum aperture position and the predetermined proximate position, the edge portions (14g, 14h, 16g, 16h; 114g, 114h, 116g, 116h; 214g, 214h, 214i, 216g, 216h, 216i) forming an n-polygonal flat-shape having a width of the n-polygon in the first direction which is larger than the width in the second direction.