PURPOSE: To obtain an assembling device which enables a quadrupole permanent magnet possessed of an intense magnetic field to be easily and precisely assembled.

CONSTITUTION: A pole piece holding piece 15, a regular octagonal base table 21 provided with a concentric stepped parallel plate whose periphery is higher than its other part and horizontal channel-type grooves e which are radially arranged at the center of each side and slide on the center of the pole piece holding piece 15, a reference pin 24 coaxially provided to the base table 21, a cylindrical dummy cell 30 which is coaxially inserted into the reference pin 24 in a slidable manner and provided with a flange vertical to its shaft and a slit provided to each of its opposed side faces in parallel with its shaft, and a pushing knob 27 which is provided to each of the side faces of the base table 21 and sends the pole piece 70 towards the center are provided.