PURPOSE: To decrease the number of originals to be prepared by providing a masking member for coloring, light shielding, or dimming on the optical path of projection of originals, and providing a member which moves automatically the masking member to an optional position.

CONSTITUTION: A masking device has the masking member 1 for projecting an arrow-shaped image upon a projection image of an original on its optical path of projection. Then, the masking member 1 is made of, for example, colored light-transmissible synthetic resin, etc., to color the projected image, and also made of an opaque material to shield only the image pickup part. The masking member 1 is fixed to a transparent belt 2 and movable to an optional position. Consequently, the masking work on a projector provided to an OHP body is facilitated and the number of originals to be prepared is reduced.