To provide a half mirror varying device which makes a stereoscopic video display device selectively usable in two-dimensional and three-dimensional video displays by vertically variably supporting a half mirror.

The half mirror 15 fixed to one set of supporting frames 22 is rotationally moved up and down around a hinge shaft coupled to a bottom end, by which the two-dimensional and three-dimensional videos may be selectively displayed. Rack gears are formed at the top ends of the supporting frames 22 and pinion gears meshed therewith are rotated at a prescribed angle, by which the angle of the half mirror 15 may be precisely adjusted.