A magnetron cathode assembly which realizes stopping of turn of an external terminal inserted into a stem insulator without increasing a manufacturing cost. A leading end of a base end axial portion of an external terminal has a non-circular section by providing a flat surface in at least one position on its peripheral surface, and a straight stop edge that fits to the flat surface thereby to carry out stopping of turn of the external terminal is provided for a terminal fitting hole of a sealing metal plate into which the leading end of this external terminal is fitted.