A radio transmitter for modulating a carrier wave in a predetermined frequency with given information to transmit the modulation signal through an antenna having a predetermined resonance frequency f1 has a frequency changing section that changes the frequency of the carrier wave. The frequency changing section changes the frequency of the carrier wave to one of the resonance frequency f1 and a frequency f1×1/n, (wherein n is a given integer of at least 2, to change transmission output power.