A radio observation device which transmits information on observation subjects has a first coil, an oscillator, a first frequency controller, a frequency-measuring device, and a transmitter. The oscillator generates signals flowing through the first coil within a certain range of frequencies. The first frequency controller is connected to the oscillator, and adjusts the oscillating frequency of the signal flowing through the first coil by controlling said oscillator. The frequency-measuring device measures a resonance frequency of a signal flowing through the first coil and a signal flowing through a second coil being provided external to the radio observation device. The transmitter transmits the information on the observation subjects through said first coil by a carrier wave which oscillates at the resonance frequency.