PURPOSE: To allow one image pickup device to cope with the measurement for different optical multiple by providing a pad in contact with the face of a cathode ray tube to a lens system and driving independently the lens system and a camera.

CONSTITUTION: Purity is measured by obtaining the position deviation of a center position 21a of an electron beam 21 with respect to a center position 20a of a phosphor dot 20 in a lens system 5. Thus, a cylinder 14 is operated to move the lens system 5 toward a cathode ray tube 1. A camera 6 is retreated backward by a cylinder 15, a hook 13 is engaged with a stopper 12 and the lens system 5 and the camera 6 are moved on a rail 9 while keeping the distance of them constant and a pad 10 provided to a tip of the lens system 5 is in contact with the face 1a of the cathode ray tube 1 and the lens system 5 is focused.