PURPOSE: To generate an electromagnetic wave in a very high frequency region and its very short pulse with high frequency and broad band by transforming a laser light beam into an electron beam, and generating an electromagnetic wave by means of Cerenkov radiation through an accerelated electron beam.

CONSTITUTION: A photoelectric transformation is performed by inputting a very short light pulse LP of a predetermined period sent from a light source OPS onto a photoelectric surface PK to perform a photo-electric transformation, the electronic pulse of the transformation output is fully accerelated by an accerelating electrode G adjacently arranged to the photoelectric surface PK and by a successively arranged electron accerelating system ACS, so that a high speed very short pulse electron beam EP bearing sufficient high energy is formed. Next, Cerenkov radiation is generated by passing the electron beam EP, accerelated up to a speed relating to the electromagnetic wave propagation velocity in the following dielectric, through a fine groove provided in an dielectric truncated cone DCO. The radiated electromagnetic wave is led through an electromagnetic wave reflecting mirror RM and a convergent wave guide type LC, in turn, so that an electromagnetic wave containing an instantaneous electromagnetic waveform can be output in a pulsive or a continuous fashion.