A small-size high-power terahertz oscillator achieves a stable oscillation in a terahertz frequency band even at room temperature. The high-power terahertz oscillator has a structure in which a bow-tie antenna is disposed on a substrate, a cavity resonator, which includes two cavities, is disposed at a power supply portion of the bow-tie antenna, and a resonant tunneling diode (RTD) is disposed along a bottom of a wall of the cavity resonator, which defines the two cavities, and stably oscillates waves in the terahertz frequency band at room temperature by using the RTD, the bow-tie antenna and the cavity resonator.