36 includes a bent off portion 41b engaging a slide edge 36a of the rotor 36 the lever 41 with its portion 41b follows the counterclockwise motion of the rotor 36 in response to the action of the spring 42 and the clockwise return movement of the rotor 36 in response to the action of the spring 39 mounted on the sector lever 41 is a section pin 43 through which the sector lever 41 turns shutter blades 45 which also serve as diaphragm blades about sector shafts 44 so an exposure aperture 46 is partly or wholly opened when an exposure is being made the arresting lever 21 is rotatable about a shaft 47 and is urged clockwise by a weak spring 48 when the print coil 37 is supplied with a current pulse of short duration the rotor 36 turns counterclockwise and strikes the arresting lever 21 at the bent off portion 21b then the arresting lever 21 executes a back and forth rotary motion i e makes a round trip thereby the pawl 21a of the arresting lever 21 catches the next tram tooth 8a thus the slide 8 advances one step for every one pulse of current supply this current supply is so short and therefore the rotor 36 moves so rapidly that the second lever 41 cannot follow up because of the inertia of the shutter blades 45 therefore the shutter blades 45 do not open the sector lever 41 has a detent portion 41a arranged to engage a position detecting portion 14b of the barrel slide 14 when the barrel slide 14 is in the illustrated position so that the shutter blades 45 are hindered from opening even mechanically after the release member 1 reaches its second step.