PURPOSE: To prevent dazzle in a working place without spoiling brightness in the entirety of a room by hanging down a blind formed of a number of flaps concavely curved upward over the full length, inside a window. CONSTITUTION: An anti-dazzle device 4 formed of a number of thin plates 5,... at least partly metalized or coated with a reflecting coating material and whose cross section is conecavely curved upward over the full length, is hang down inside a window 3. The longitudinal peripheral edges 5a facing the indoor side 1, of the thin plates 5,... are made the same height with the outside longitudinal peripheral edges 5b or higher than that. When the anti-dazzle device 4 is not required, it is pulled up and contained in an upper housing position 4. In this way, only slight light is allowed as incident ray at the under part of the nearly horizontal critical angle and quantity of light as much as possible is transmissible at the other parts.