PURPOSE: To improve a current detection sensitivity by increasing the magnetic efficiency of a magnetic circuit and to improve the precision in positioning of a core by adopting a structure of fitting the fore end part of the core in a recessed part of a base.

CONSTITUTION: A ferromagnetic core 4 of which a part of a magnetic path is opened, a spool 6 which is provided around on one side of this core 4 and on which a coil 3 is wound, and a Hall element 2 disposed in the vicinity of the opened part of the core 4, are mounted on a base 1. Besides, a detecting circuit which amplifies and delivers an output of the Hall element 2 is formed also on the base 1. On the other hand, the base 1 has a recessed part 5 formed in the rear so as to lessening a core gap, and the tip part of the core 4 disposed on the lower side of the base 1 is fitted and fixed in the recessed part.