PURPOSE: To form a return core having appropriate thickness by arranging a plurality of pairs of pole piece core in the axial direction of a measuring pipe and feeding back the flux through a common return core thereby making uniform the flux density distribution.

CONSTITUTION: Two pairs of pole piece cores 17A, 18A are arranged vertically on the surface in the axial direction of a straight pipe part 15A extending perpendicularly to a line connecting a pair of detection electrodes 16A. Excitation coils 19A, 19B, 20A, 20B wound around the pole piece cores are interconnected such that the exciting currents flowing through the excitation coils induce fields in same direction. A tubular return core 21 is applied to the outside of the cores 17A, 18A and the coils 19A, 19B, 20A, 20B thus constituting the return path of flux. With such structure, a flux flowing into the core 21 is dispersed into fluxes B3, B4, B5 and B6 which flow into the cores 17A and 18A. Since the flux density distribution is made uniform even in the vicinity of joint between the cores 17A, 18A and the core 21 similarly to the central part of the core 21 and flux saturation is prevented, the core 21 can be formed with uniform thickness.