A tube-shaped electroconductive member is attached to a shielding casing from the inside of the shielding casing so as to surround an opening formed in the shielding casing, and two slits are formed in an axial direction of the tube-shaped electroconductive member so as to extend from an end position thereof far away from the opening, thereby preventing a noise component having a specific frequency emitted as radiation noise from the opening formed in the shielding casing which functions as an slot antenna.