Abstract: A cassette retaining device comprises a pair of retaining mechanisms disposed respectively on both sides of the top surface of a sample table and pressing respectively on both sides of the top surface of a cassette to hold the cassette in said predetermined position between said retaining mechanisms and the sample table. Each retaining mechanism includes a presser capable of moving between a first position where the presser presses on a side portion of the top surface of the cassette to hold the cassette and a second position where the presser is separated from the top surface of the sample table to allow the cassette to be inserted or removed from between the presser and the sample table, a rocking arm rockable around a first pin relatively to the sample table and bearing the presser so that the presser may rock around a second pin parallel to the first pin, and a spring urging the rocking arm to rock in one direction, thereby locating the presser in the first position.