A screen unit includes: a screen member; a first support member which supports one end of the screen member; a second support member which supports the other end of the screen member; a stretching and contracting device which expands the screen member; and a tensioning member disposed on the side opposite to the side where the screen member is provided with respect to the stretching and contracting device to apply tension in a direction substantially parallel with the direction from the first support member to the second support member.