14 compares the processed signal with a reference signal from a reference signal source 15 and a signal processing circuit 16 calculates appropriate values of electric currents for the electromagnets 4a and 4b based on the compared results an amplifying circuit 17 amplifies electric currents to the electromagnets according to the calculated values so as to control the magnitude of the magnetic forces generated from the electromagnets 4a and 4b to thereby equalize the distances g 1 and g 2 with each other and thus prevent the sample carrier 3 from contacting the inner surface of the reaction chamber 2 further as described before with reference to fig1 the electromagnet pair 4 is fixed to the movable support 6 and the sample carrier 3 is held by the magnetic attractive forces of the electromagnets 4a and 4b so that the sample carrier 3 can be magnetically displaced and positioned in response to the corresponding displacement of the electromagnet pair 4 i e the displacement of the support 6 in parallel manner in the above described embodiment the electromagnet pair 4 and position sensors 5 are aligned in a horizontal direction with respect to the reaction chamber 2 and the sample carrier 3 however these elements can be aligned in any linear direction such as.