PURPOSE: To provide a slit disc which enables the holding of an even negative high pressure level, by being combined with a rectangular slit in such a manner as to make the length-wise direction the rectangular slit align the radial direction thereof while first and second slits are arranged additionally to eliminate a peak otherwise generate with a sharp rise in the negative high pressure.

CONSTITUTION: Luminous flux from a crossing section between the first slits 4-1 and 4-2 and the second slits 6-1 and 6-2 of a slit disc 2 and a rectangular slit 10 passes in a almost constant quantity without interruption to always make light enter a photo multiplier tube 14 form monitoring in an almost constant quantity. This eliminates the developing of a sudden rising peak in the negative high pressure, even with an automatic correction of the negative high pressure which would be applied for other photo multiplier tubes 22 and 24 by a dynode feedback using the output thereby enabling measurement under an almost even negative high pressure level.