PURPOSE: To realize a highly accurate balance test by applying a magnetic field, variable in the rotational direction, to an object to be balance-tested.

CONSTITUTION: A wheel set A to be tested is mounted on rails 10a, 11a at the central position thereof and then the rails 10a, 11a are lowered through operation at an operating section 17. The axle 2 is mounted, at the opposite ends thereof, on the bearing parts 13a, 14a and a reference mark for detecting the rotational speed is put on the axle 2 at one point thereof. Subsequently, linear motor units 16a1-16a4 are lifter through operation at the operating section 17 and the relative position to the wheels 1a, 1b is adjusted. Currents are then fed from a linear motor drive circuit to the units 16a1-16a4 thus generating a field variable in Z-direction. Consequently, the wheel set A begins to rotate and the r.p.m. thereof increases gradually. When a preset r.p.m. is reached, power supply is interrupted to bring the wheel set A into free running state. Vibration at the bearing part 13a, 14a is detected by a sensor under that state and stored, in relation to the rotational speed of the reference mark, in a memory within an arithmetic circuit. Finally, the balance amount is determined at each angle based on the stored data and displayed 18.