To prevent film pressure spring which is pressed in a fitting hole from coming out.

A plate 27d is formed by bending an end part of a main body 27a so that the head end side in the direction of pressing into the spring fitting hole 28 has width W2 narrower than the width W1 of the spring fitting hole 28 and the tail end side has wider width W3. When a plate 27d part is pressed into the spring fitting hole 28, the plate 27d elastically deforms and a lock arm 27e and an engagement arm 27f are energized toward the side face of the spring fitting hole 28. With the engaging force, a 2nd film pressure spring 27 is held in the spring fitting hole 28. After the plate 27d is pressed into the spring fitting hole 28, the tip of a fall preventive claw 27g intrudes into the side face and the 2nd film pressure spring 27 never comes out. The 2nd film pressure spring 27 can easily be fixed only by being pressed into the spring fitting hole 28 and never comes out.