A casing contains a circuit substrate. A signal electrode for transmitting and/or receiving an electronic field signal is arranged on a first surface side of the casing. The signal electrode is electrically connected to a conductor, which is an apparatus component, of the circuit substrate. A battery and a reference electrode having an electric field reference potential, which are apparatus components, are arranged on the second surface side of the casing. The sum (Cst) of a capacitance between the signal electrode and the transmission medium and a capacitance between the apparatus component and the transmission medium generated while the first surface is facing the transmission medium is approximately equal to the sum (Cgt) of capacitances between the apparatus components and the transmission medium generated while the second surface is facing the transmission medium.