PURPOSE: To obtain a small size and multi-functional block filter by incorporating a current detecting function utilizing a magnetic sensor in the block filter.

CONSTITUTION: An L1 and a C1 in a high impedance circuit form a parallel resonance circuit resonated at a carrier frequency f0. The L1 is an inductance having a large current capacity to which a current of commercial power supply flows. A magnetic core of the coil 11, a ferrite core 12 and a core gap 13 preventing magnetic saturation by a large current I are formed. Moreover, a low impedance circuit is an LC series resonance circuit and its resonance frequency is coincident with the f0. A large block level is obtained to a carrier signal by both resonance circuits. Then a Hall element is sticked to a core end face of the core gap 13 as a magnetic sensor and the magnetic flux generated by a current I flowing to the coil 11 exists in the core 12 of a high permeability, and a magnetic field vertical to the Hall element 14 is activated and highly sensitive current detection is attained.