The present invention relates to a composition for liquid crystal alignment layer, a preparation method of liquid crystal alignment layer using the same, and an optical film comprising the liquid crystal alignment layer. Therefore, the deterioration of the fluorochemical surface modification film of the outer circumference of the lens barrel 10 does not proceed earlier at a local region, so that the premature wetting of the outer circumference of the lens barrel 10 with the magnetic fluid 4 can be prevented to elongate the lifetime of a light shielding device 1. The sum of the longitudinal components of the opening ratios of s adjacent in the lateral direction is constant. When gas is released by the gas generator the piston deforms and wedges into a space between the outer peripheral surface of the adjacent force transmitting member and the inner peripheral surface of the tube.