PURPOSE: To measure luminescent spot density at numbers of points specified on a fluorescent screen automatically with good operation efficiently, by providing an arithmetic device which supplies desired indications to a machine system and a luminescent spot controller while observing the outputs of photosensors.

CONSTITUTION: An optical system 1 is moved to measurement position coordinates and the arithmetic device 11 selects an electron gun which is not measured and supplies an entire-surface raster signal to the luminescent spot controller 19 at the same time to make the fluorescent screen monochromatic uniformly. Further, optical fiber changeover switches 131 and 132 are set in up-down or left- right directions to input the output of one of two photosensors 121 and 122 to the arithmetic device 11, and while two-axial movement system 3 is moved finely almost within the range of the pitch of fluorescent materials, an automatic focus system 2 is moved so that the output of the other sensor, e.g. 121 is maximum. Then while the outputs of those two photosensors 121 and 122 are inputted to the arithmetic device 11, the two-axial movement system 3 is moved so that both photosensors 121 and 122 have the quantities of light greater than set levels.