[en] Beryllium has been utilized as a moderator and/or reflector in a number of material testing reactors because of, low parasitic capture cross section for thermal neutrons and good neutron elastic scattering characteristics. Beryllium frames and beryllium reflectors, which have been utilized as neutron reflector in Japan Materials Testing Reactor, were fabricated with beryllium metals of S-200F grade. Especially, the beryllium frames is necessary to exchange every fixed period. Therefore, preliminary irradiation test of beryllium metals (S-200F and S-65C) was performed from 162nd to 165th cycles of JMTR operations as a part of development on life time extension of beryllium reflectors. The post irradiation examinations were carried out for these irradiated beryllium metals. In these PIEs, size change of the irradiated beryllium was measured by high accuracy size measurement device. This report is described development of the high accuracy size measurement device and its performance. (author)