A screen unit includes: a rectangular screen; fixing assist members provided on the screen along two opposed sides of the screen; first supporting units fixed to two sides of the screen different from the two opposed sides along which the fixing assist members are provided; second supporting units fixed to the screen via the fixing assist members; a first tensioning unit which applies tension in the direction of separating the first supporting units away from each other; and a second tensioning unit which applies tension in the direction of separating the second supporting units away from each other.