PURPOSE: To improve the roundness of approximate circles by superposing multiple sheets of movable blades of large degrees of bending which are supported pivotally at one end at an equal spacing on the same circumference and creating the contours of the proximate circles by part of the inner edges on one end side of the respective movable blades and part of the inner edges on the other side, with respect to the shutter for projectors.

CONSTITUTION: 24 Sheets of arc-form movable blades A of large degrees of bending are used. The α part and β part of their inside edges cover 1/24 of a circle. More specifically, the A1β, A2β... where 1b of the β part covers 1/24 of the circle are added, in addition to the loci of the portions A1α, A2α... where 1/24 of the circle is covered by the α part of the respective blades A1, A2... The 1b part of the time when the circle of (a) is formed positions between the 8a part and 9a part of the other movable blades A8, A9 to smooth the angle that the both movable blades A8, A9 assume. Hence, the A1α, A2α... substantially cover 1/48 of the circle and the A1β, A2β... also cover the 1/48 of the circle, respectively. Hence, each movable blade covers just 1/24 of the circle but the circle close to a true circle may be obtained.