A microwave tube has a configuration in which the connection between an anode electrode and the helix is cut, the helix is grounded, and a current suppression element is provided between the anode electrode and ground voltage for suppressing the continuous time interval of the flow of an excess current to the helix that is caused by discharge to a time interval range that does not threaten damage to the helix, i.e., to a time interval range in which an alarm signal is not supplied.