A radio IC device that is highly efficient in transmitting signals from a radio IC to a radiation pattern includes a radio IC chip arranged to processes radio signals, an auxiliary electrode pattern that is connected to the radio IC chip and includes a loop-shaped electrode, and a radiation electrode pattern that includes a magnetic-field radiation electrode having a resonant frequency and an electric-field radiation electrode having a resonant frequency. The loop-shaped electrode of the auxiliary electrode pattern is coupled to a maximum-voltage section of the magnetic-field radiation electrode through a capacitor.