Abstract: A magnetron for a microwave oven comprising a spacer having a disc shape fitted between a lower pole piece and a F-seal supporting the lower pole piece. The spacer has a pair of lead holes through which a center lead and a side lead extend, respectively. At the upper surface of the space, a metal coating is formed which defines an attenuation cavity. The provision of the attenuation cavity makes it possible to remove effectively leaked microwaves. Since the spacer is firmly fitted between the lower pole piece and the F-seal, it is also possible to avoid a lateral vibration of the lead assembly.