PURPOSE: To prevent efficiency of a millimetric wave generating device from lowering by forming a beam tunnel section provided between an electron gun section and a resonance circuit along a track of hollow-like electron beams, and making the ratio of a radius of the beam tunnel section to a radius of the hollow-like electron beams constant.

CONSTITUTION: When large current hollow-like electron beams 10 are moving from an electron gun section 1 to a high frequency resonance circuit within a mirror field generated by a main solenoid 8 and an electron gun solenoid 9, potential of the electron beams 10 falls. But, when the beams 10 of a radius RB and thickness δ are surrounded by a conductive wall of a tunnel section of a radius RW on the same axis, a potential fall of the electron beams can be suppressed at 10% or less by setting the radius RW at 1.4 times or less of the radius RB. That is, a beam tunnel section 2 is formed along a track of the beams 10 and the ratio of RW/RB is made constant, thereby the potential fall of the beams 10 passing through the tunnel section 2 can be lessened.