PURPOSE: To simplify the structure, by providing a means which holds aperture blades in an intermediate aperture position when two bobbin coils are not excited together.

CONSTITUTION: An aperture blade 1 is fitted to a shaft 4 of a supporting base 3 by a hole 1a and is fixed pivotally to the supporting table freely turnably. An aperture blade 2 is fitted to a shaft 5 of the supporting base 3 by a hole 2a and is fixed pivotally to the supporting base 3 freely turnably. An aperture blade driving pin 6 which is operated in directions C and D by a plunger is engaged with respective notches 1b and 2b of aperture blades 1 and 2, and the size of an aperture part S between aperture blades 1 and 2 is changed. A neutral spring 7 is stretched to respective long holes 1c and 2c of aperture blades 1 and 2 and holds aperture blades 1 and 2 so that the size of the aperture part S is normally in the intermediate aperture state.