PURPOSE: To eliminate an irregularity in exposure by supporting at least either of slit forming blades of a leading and a trailing curtain pivotally on an arm so that the slit width is wider at the tip side of the blade than at the arm coupling side.

CONSTITUTION: The slit forming blades 11 and 21 of the leading curtain 10 and trailing curtain 20 are supported pivotally on the arms so that the slit edges 11E and 21E of a leading slit forming blade 11 and a trailing slit forming blade 21 which face each other slant at θ1 and θ2 to an aperture image plane 50 and the slit width is wider at the blade tip side which is not supported pivotally than at one end (coupling side) which is supported pivotally on the arm. Namely, the slit forming blades 11 and 21 are widened in slit width at their blade tip sides, so even if the blades are put in operation narrowing down the blade tip sides in fast shutter speed operation, the slit width in slit exposure is nearly equal between the blade tip side and coupling side. Consequently, the irregularity in exposure is eliminated.