PURPOSE: To measure the two-dimensional incident position of corpuscular beam with high sensitivity, by bringing the length of each fiber forming an optical fiber group to a different length at every unit element of a two-dimensional scintillator.

CONSTITUTION: Corpuscular beam from a γ-ray source S is incident to a two- dimensional scintillator 1. The beam emitting means of the two-dimensional scintillator 1 is connected to a position/time converting connection means 3 comprising an optical fiber group by a beam receiving means. Optical fibers 301, 302... 30n are provided so as to be made successively long per a fundamental unit length Δl and, when the length of the optical fiber 301 is set to (l), that of the optical fiber 302 comes to l+Δl and that of the optical fiber 30n comes to l+ (n-1)Δl. Beam connected to a photoelectric surface 4a is amplified by a photomultiplier tube 4 and an amplifier 6 through the position/time converting connection means 3 and connected to an oscilloscope 7 and observed.