The oscillator with many active devices using higher mode of planar circuits for power combinig of microwave/millimeter wave band were investigated. For the fundamental study, configuration of the oscillator with TM_21 mode of circular resonator and Gunn Diode have been studied. In experiments at X-band for the oscillator with one element or two elements, power of two elements case has obtained about two times that of one element. As a result, the power combining method proposed here is confirmed both experimentally and theoretically.