Small-signal negative conductance of a field emission monotron was calculated, which was considerably higher than that of a thermionic monotron due to strong bunching of emitted electrons in a field emission. The amplification constant of 28dB/mm is achieved at THz wave region by applying the monotron to a distributed amplifier and is sufficient to overcome power loss in a transmission line. In addition, a wide band frequency tunable electromagnetic wave source may be developed at THz wave region by providing a low loss microstrip line resonator with field emission array due to the gate voltage dependent phase constant of the line.