A buckle pivotally arranged in a movable clamp 12 of a first tube holder 1 is attached with play to a buckle 120 pivotally arranged in a movable clamp 82 of a second tube holder 2. When tubes are held in the first and second tube holders 1 and 2, release of the tubes from the holders 1 and 2 is prevented by a movement of a plunger 203 in correspondence of excitation and demagnetization of a solenoid 202 for a predetermined period of operation of the apparatus after holding the tubes (i.e., for a period required from the locking of the buckle 120 until the completion of moving-down of a wafer holder).