Provide a nuclear reactor power monitoring technology for enhancing the monitoring accuracy and reliability in nuclear thermal hydraulic stability.A power monitor 30 includes a first calculation unit 42,52 configured to calculate a first stability index based on time series data indicating power oscillation in nuclear instrumentation signals outputted from a plurality of nuclear instrumentation detectors 31; a first determination unit 44,53 configured to compare the first stability index and a first reference value and determine whether nuclear thermal hydraulic stability of the reactor core is stable or deteriorated; a second calculation unit 46,54 configured to calculate a second stability index of the reactor core based on the time series data when the deteriorated state is determined in the first determination unit 44,53; and a second determination unit 47,55 configured to compare the second stability index and a second reference value and determine whether to perform suppressing operation of the power oscillation.