PURPOSE: To obtain an accurate stop position by running a driving motor forward and backward and stopping the same at the time of moving a cassette upward and downward for as much as one sheet of samples.

CONSTITUTION: When a motor 1 which moves a cassette in which samples are contained in vertical directions revolves, a disc 3 revolves. When a round hole 21 for detecting crosses the optical path constituted of a light emitting element 29 and a photodetector 30, it turns out that the cassette lowered for as much as one sheet of the samples. The output of the photodetector 30 is conducted to a photoelectric converting circuit 31 and a clock pulse generating circuit 32 and holds a holding circuit 25 at a backward running level; therefore, the output of the circuit 23 inverts and a negative driving voltage is applied to the input of the motor 1 by a driving circuit 26, so that the motor 1 runs backward. When the output again crosses the round hole 21 after the backward running, the output signal of the circuit 31 is conducted to the circuit 23 and the motor 1 is stopped only during the period while the output of the circuit 25 is holding the backward running level.