PURPOSE: To lengthen the mounting length of a compression coil spring for energizing a shutter and to give a degree of freedom to the specification of the spring so as not to give an unnecessary load to the shutter by providing a bend at one end of a guide groove for the

CONSTITUTION: By forming the bend 12EA at one end of the guide groove 12E for the compression coil spring 4, the guide groove 12E is lengthened by the amt. of the length of the bend 12EA, so as to allow the mounting length of the compression coil spring 4 to be lengthened accordingly. Consequently, the degree of freedom of the specification of the compression coil spring 4 such as the number of turns of the spring and its coil diameter, etc., can be increased. Then, since the spring length of the compression coil spring 4 under the fully opened state of the shutter 13 is longer than the conventional, variations of spring force at the compressing time and at the expanding time of the spring 4 can be diminished, so that the shutter 13 can be prevented from receiving an unnecessarily large load from the compression coil spring 4.