PURPOSE: To improve the resolution and uniformity in sensitiveness by providing optical absorbing layers on the front and back of a light guide.

CONSTITUTION: The front 41 and back 42 of the light guide 4 on the center line of a photo-electron multiplier 5 are formed with optical absorbisng layers 81 and 82, the latter of which is sized idential to the shadow which is projected upon the back by the front layer 81 when a light source P is positioned at X=0. In this case, the shadows to be projected upon the multiplier 5 are overlapped completely at the two sides 81 and 82 to assume the minimum area. If the light source is shifted from the position of X=0, the area of the shielded shadows is gradually increased. This area becomes twice as that of the minimum when the source P comes to the position P1.