A magnetron can be inspected with high accuracy. A life of the magnetron is determined on the basis of a comparison between a current parameter, which indicates a current status of the magnetron and is obtained from the one or more measurement values for specifying a current status of the magnetron at a time point when a time period having a predetermined duration or more has elapsed after generation of a high frequency power by the magnetron is started, and a difference between a power of a progressive wave and a set power is equal to or lower than a first predetermined value and a power of a reflection wave is equal to or lower than a second predetermined value, and an initial parameter, which indicates an initial status of the magnetron and corresponds to the current parameter.