An assembling structure is provided which is capable of greatly reducing the material cost, is easy-to-assemble, and can be made smaller. The sensor body is provided on a lower face with a protrusion, which comes into contact with an upper face of the busbar when the sensor body is mounted on the busbar. The busbar is provided with a through-hole passing through the busbar in a vertical direction. A leading end of the protrusion, which passes through the through-hole when the sensor body is mounted on the busbar, is heat-fused and adheres to the circumference of the through-hole.