To provide a compact field emission cathode and an electromagnetic wave generating device of simple constitution capable of effectively generating electron beam modulated by random frequency within a wide band centering at terahertz band, ranging from submillimeter wave band to far infrared band.

Electrons are excited by irradiation of an ultrashot pulse laser beam or two kind of laser beams with frequency different from each other from a laser light source 12 to a needle-like projection arranged on the surface of a cathode member 11. These electrons are emitted from the surface of a cathode by quantum mechanical tunnel effect in strong field due to DC power 15 for a field emission connected between the cathode member 11 and a gate electrode 14. Electromagnetic wave is generated by interaction between the electron beam and electromagnetic wave in a high frequency circuit. The surface of the cathode member 11 can be constituted by p-type semiconductor and intrinsic semiconductor capable of electronic excitation to conduction band due to luminous excitation, semiconductor heterojunction structure capable of spatial selective excitation of electron, quantum well and super latice, quantum dot capable of selective excitation due to energy of irradiation light source, fullerene, low dimentional superstructure such as a carbon nano tube.