To eliminate or simplify a circuit for correcting a raster distortion optically caused in a three-cathode-ray-tube display device so that the external shape of a three-tube back projection type display device is finished to be compact.

In two right and left cathode ray tubes of the three-cathode- ray-tube back projection type display device, the optically caused raster distortion is not corrected by correction by the conventional electric circuit. By changing deflected magnetic field distribution, the initial optical raster distortion is corrected.