PURPOSE: To retard unnecessary etching when a sample is made thin by etching in order to allow observation thereof by means of a transmission electron microscope.

CONSTITUTION: A few drops of an etching liquid 17 is fed, by means of a filter, to the upper part of the hollow section 12a of a tubular etching liquid holder 12 having inner diameter of 1.4mm and outer diameter of 3mm. Consequently, the etching liquid 17 is held on the upper side of the hollow section 12a while being swollen by the surface tension. When a thin film sample is mounted on the upper face of the holder 12 through a substrate 14, the lower surface of the substrate 14 is etched in the center thereof. In this regard, the production of the vapor of etching liquid 17 and leakage of the thin film sample to the upper face of the thin film 15 due to the surface tension of the etching liquid 17 can be reduced significantly and etching on the upper face of the thin film 15 is retarded.