Preparing a thin-film specimen adapted for TEM (transmission electron microscopy) observation. A high-brightness pixel extraction unit extracts high-brightness pixels which form a specimen image taken by an imaging unit. The intensities becoming greater than a given threshold value as the specimen is thinned. A decision unit makes a decision as to whether the high-brightness pixels extracted by the high-brightness pixel extraction unit form a continuous sequence of pixels whose number is in excess of a given number on the specimen image. If the decision is affirmative, the decision unit sends a signal to an ion gun control unit to stop the ion-beam irradiation of the specimen.