PURPOSE: To perform a rapid and hig-precise measurement of a white balance over a whole fluorescent screen through a simple structure, by mounting a rotary oval mirror and an actinometer in front of a color picture tube.

CONSTITUTION: A rotary oval mirror 23, having a rotary oval mirror surface, is constituted such that a center F1 of a spherical fluorescent screen 21 of a color picture tube, and a center F2 of an actinometer 22 forms the other focus. The mirror 23 is mounted, and the actinometer 22 is rocked against the rotary oval mirror 23 by means of a rocking mechanism 24. The light volume measurement of the actinometer 22 is read by evaluating device 25 to evaluate evenness of emission of a spherical fluorescent screen 21 of the color picture tube 20.