An electronic device which includes a radio wave impermeably cylindrical case, a radio wave permeable cover covering an opening in one end of the case, a radio wave impermeable cover covering an opening in the other end of the case, and a circuit board and an antenna, both arranged between the two covers in the inner space. The antenna is connected to the board and includes an elongated core and a coil wound around an intermediate portion of the core. Magnetic flux introducing portions are provided on both end portions of the core, a radio wave permeable partition plate is arranged between the permeable cover and the antenna in the inner space, and a pair of flat and magnetic members are arranged between the partition plate and the antenna in the inner space and are magnetically connected to the flux introducing portions.