PURPOSE: To assure the transmission route of γ rays and to surely measure a level by deforming a bundling shape of coils windings of electromagnetic coils symmetrically disposed with magnetic poles and providing notches in which a resin is filled.

CONSTITUTION: The respective magnetic poles are symmetrically disposed on the circumference and the coil windings 13 are symmetrically deformed in the bundling shape of the parts 13a corresponding to the γ ray transmission route 4 to assure the transmission route 4 of the γ rays. The parts 14a corresponding to the transmission route 4 of a coil case 14 are notched and the resin 15 of light mass is filled into said parts. The resin 15 is preferably the resin having 1 specific gravity which is the same mass as the mass of the water in the other part where the γ rays pass, for example, the cooling water passage. The attenuation of the γ rays is thus prevented and the stable level measurement is assured.