PURPOSE: To make a measuring instrument thin by providing a holder, which detains one end of each stretched band, on the side of an internal core and providing a spring member, which detains the other end of each stretched band, on the side of a frame-shaped coil and supporting the frame-shaped coil on the holder through each stretched band.

CONSTITUTION: An external core 1, an internal core 3 provided with the same axis as the external core 1 through attaching metallic tools 2, and a coil device 5 which is supported by the internal core 3 turnably in a magnetic gap 4 formed with external and internal cores 1 and 3. The external core 1 is formed into a cylindrical shape with high-permeability metallic materials, and a ring-shaped flange 6 is provided on the circumference of the external core 1. The attaching metallic tool 2 consists of a metallic band which has two sides bent at right angles and has an L-shaped section, and its one side is fixed to the flange 6 by a screw or the like, and the internal core 3 is attached to the other side by adhesion or the like.