First and second electrically insulating substrates are joined with each other at respective joining faces thereof. Each of the first and second insulating substrates has an annular groove at the joining face, and a plurality of through holes along outer and inner peripheries of the annular groove. An annular core is mounted in the annular groove. A cylindrical connection is formed in each through hole, and a radial connection is formed on an outer surface of each insulating substrate so as to connect opposite cylindrical connections. A toroidal coil is formed by serially connecting cylindrical connections and radial connections. An IC chip is mounted on the outer surface of the first insulating substrate, and connected to the toroidal coil and electronic part. The toroidal coil and the electronic part are coated with an electrically insulating material.