PURPOSE: To allow the formation of a sample having a wide observation range by thinning the sample prior to ion milling.

CONSTITUTION: A sample of silicon substrate is cut into a sample piece (silicon) 2 of 5mm square which is then polished as thin as about 30μm from the side opposite to the observing surface 1. The surface 1 is entirely applied with a photoresist film 3 which is then removed at the end parts of the sample piece 2. It is then immersed into an HF based silicon etching liquid and the removed part of the film 3 is etched to several mm thick. After removing the film 3 entirely, silicon oxide 4 is deposited on the entire surface including the etched surface. It is then bonded, on the silicon oxide 4 side, to an abrasive board such as a glass plate and the sample piece 2 is abraded, on the rear thereof, until the silicon oxide 4 is exposed. A reinforcing ring 5 is then bonded through an adhesive to the test piece 2 thus removing the silicon oxide 4. Furthermore, the thickness of the test piece 2 is set at several hundreds nm, as required, by ion milling or the like thus preparing a sample.