The present invention relates to a thermal storage unit using an electromagnetic wave of solar radiation, more specifically, to a thermal storage unit that converts and stores energy of an electromagnetic wave of the solar radiation which is irradiated on to the earth's surface of a large area in the form of a thermal energy in an electromagnetic wave state and uses the stored thermal energy whenneeded. The present invention comprises a radio wave reflector focusing an electromagnetic wave of solar radiation, a waveguide leading and guiding the collected electromagnetic wave energy, an energy conversion and storage unit converting and storing the electromagnetic wave energy that is transmitted along the waveguide in another form of thermal energy, and an energy supply unit utilizing the stored thermal energy in the energy conversion and storage unit for general use. Even though the radio wave reflector and the energy conversion and storage unit are installed at a distance considerably far from each other, the thermal storage unit of such configuration can improve solar radiation use efficiency to a maximum, by guiding, through the waveguide, the solar radiation having been focused to the radio wave reflector to the energy conversion and storage unit in an electromagnetic wave state.