By comparing a digital image signal obtained by inverting a digital image obtained from one of the scan lines by {HEAD ~ 180 degrees} with {TAIL ~ a digital image signal} obtained from the other of the scan lines and detecting the tail 175 appearing as noncoincidence on both sides of the pattern 171, the charge up phenomenon which has appeared in the pattern 171 can be grasped 
with
By comparing a digital image signal obtained by inverting a digital image obtained from one of the scan lines by { HEAD ~ 180 degrees } with { TAIL ~ a digital image signal } obtained from the other of the scan lines and detecting the tail 175 appearing as noncoincidence on both sides of the pattern 171 , the charge up phenomenon which has appeared in the pattern 171 can be grasped
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