portion 41 of the driving shaft 24 because the force f 0 perpendicularly exerted by the spring on the center of rotation of the driving shaft 24 fixes the driving shaft 24 at a predetermined angle when the driving shaft 24 rotates to approach the predetermined angle for example at the location shown in fig4 b where the motor stops driving the force f 0 exerted by the spring pushes against the inserted element 52 such that the inserted element 52 comes into contact with a contact point 411 on an outer circumference of the driving shaft 24 close to the indented positioning portion 41 through an arc shaped guided portion the contact point 411 makes an angle of deflection with the force f 0 such that the force f 0 produces a component of force f 1 driving the driving shaft 24 to rotate and reach a stable positioning state as shown in fig4 a in other words when the driving shaft 24 rotates to approach the specify angle the driving shaft 24 automatically becomes positioned fig5 a to 5e illustrate variations of the resilient member made according to the present invention the inserted element 52 is formed in the shape of a cone thereby forming the guided portion with an inclined plane on a top thereof as shown in fig5 a while the indented positioning portion 41 has a square shaped cross section while the inserted element 52 is a cuboid having a lead angle as shown in fig5 b referring to fig5 c a plurality of the indented positioning portions 41 are formed upon the driving shaft.