PURPOSE: To prevent the image of dust from being photographed even if the dust sticks to a screen by hermetically sealing an image receiving tube by a screen cover up to the position substantially apart from the fluorescent plate thereof.

CONSTITUTION: The screen cover 11 consisting of a light transmittable material is provided freely attachably and detachably to and from a vacuum vessel 3 apart at a substantial distance l from the fluorescent plate 6 so as to hermetically seal the screen 2. The front face of the screen cover 11 is formed plane or to a specific thickness and small curvature to prevent the generation of a lens effect. Even if the dust P sticks to the screen cover 11, the fluorescent plate 6 has the substantial distance l from the screen cover 11 and therefore, the image PS of the dust P is formed to the position apart from a film 10. Only the image by the fluorescent plate 6 is thus formed on the film 10.