To provide a teraherz oscillation element which eliminates the leak of even electromagnetic waves generated by a frequency-variable oscillation element in a frequency band having a comparatively broad bandwidth from a slotted line over its oscillation frequency range, thereby generating high efficiency and high power electromagnetic waves.

Multi-step stubs are provided on both ends of a micro slotted antenna having an active device composed of RTD (Resonant Tunneling Diode), etc. to reflect electromagnetic waves in a frequency band having a comparatively broad bandwidth from the multi-step stubs. The multi-step stub circuit reflects leaked parts of the electromagnetic waves generated by the active element to return the reflected parts to the active element, resulting in a broad band high power oscillation output. If the active element used in the oscillation element is a frequency-variable element, a high power oscillation output corresponding thereto is obtainable.