To remove a produced amorphous layer, and to prepare a sample suitable for laminated structure analysis of film thickness analysis or the like, by executing a wet chemical etching by using an optional solution after a focused ion beam work in preparing the sample.

Sputtering is executed by irradiating a focused ion beam on both sides of an observation region 6 of a sample, to thereby execute film-thinning of the observation region 6 until observation by a transmission electron microscope becomes possible. In this case, crystals inside the observation region 6 are changed into the amorphous state up to the depth of several ten- to several hundred- from the surface of a work face, by receiving the influence of extension in the vertical direction of the focused ion beam irradiated to the sample. Then, after the focused ion beam work, wet chemical etching is executed to remove the amorphous layer. The wet chemical etching is executed by a chemical reaction between a solution and a sample, after immersing the sample into the solution. As the solution, for example, in case of GaAs-based compound semiconductor, acid solution, such as phosphoric acid, hydrochloric acid or the like, with a concentration of 0.01-0.1 wt.% is used, especially phosphoric acid being preferable.