A reader antenna is connected to an interrogator. Conductors are arranged at two places at the opposed marginal edge portions of a radiation plane of the reader antenna along a radiation direction of an electromagnetic wave from the reader antenna. The conductors raises the electric power density above the reader antenna. A storage case is set above the reader antenna and has many items arranged at narrow intervals with radio tags attached thereto. The storage container has the radio tags arranged substantially parallel to the polarization direction of the reader antenna. By doing so, adequate drive power is applied to the radio tag by an electromagnetic wave from the reader antenna.