PURPOSE: To obtain high-power and high-frequency output easily by arranging a plural number of tubular sphere body provided with an electron gun, a resonance cavity, a collector and the high frequency output part, inside a magnetic field coil.

CONSTITUTION: A magnetic field coil 30 focusing electron beams and the main magnetic field 32 accelerating beams are arranged while being divided into some pieces as occasion demands so as to properly regulate the distribution of the magnetic field. Inside said magnetic field coil, for instance, seven pieces of the tubular sphere body 10 consisting of an electron gun 12, a resonance cavity 14, an electron beam collector 16 and the high frequency output part 18 are arranged so as to take out seven pieces of high frequency outputs by using one set of magnetic coils. Accordingly, even when required to make the inside diameter of the resonance cavity small by raising frequency, the high output can be obtained, because the magnetic coil is so made as to enable a plural number of tubular sphere body to use the magnetic coil in common.