PURPOSE: To make it possible to measure the sensitivity and position resolution of a position detector preliminarily easily and correctly by newly providing an electron beam source for testing and testing the characteristics of the position detector by electron beams from this instead of scattered proton beams.

CONSTITUTION: An ion source 1, a magnet 2, an acceleration/deceleration tube 3, a sample 4, a position detector 6, etc., are in vacuum, and an electron beams source 8 is provided between the acceleration/deceleration tube 3 and the magnet 2 in addition in such a disposition that electron beams fly toward the magnet. The electron beams from the electron beams source 8 go through a slit of a collimator 5 to get into the magnet 2. A flux density B is produced perpendicularly to the surface at the magnet 2, so the electron beam has a semi-circular orbit to get into the position detector 6. The dispersion of sensitiveness by the position and the resolution can thus be determined for the position detector 6, and because the strength of the electron beam can be increased freely, while there are only a small number of protons scattered, time required for measuring can be reduced.