A power system stabilizing system configured to be applied to a power system, the power system configured to include a plurality of feeders including circuit breakers, a plurality of nodes, a plurality of generators, and a plurality of loads, the power system stabilizing system comprising: a first interface portion configured to receive as inputs thereof, for every plurality of circuit breakers, information associated with a voltage and a power flow close to the plurality of circuit breakers concerned, and information associated with a protection relay of the circuit breakers for deciding an opening operation of the plurality of circuit breakers; a second interface portion configured to receive as an input thereof the information associated with a voltage and a power flow containing information associated with measurement time measured in the power system; state estimation calculating means configured to carry out power flow state estimation calculation for every node of the power system by using measurement information, associated with the voltage and the power flow, from the first interface; grid fault judging means configured to judge a grid fault from the information associated with a protection relay from the first interface portion; circuit breaker opening instruction computing means configured to compute and to output a breaker opening instruction by using information in a control table and by using a fault information from the grid fault judging means; characterized by reliability judging means configured to compare power flow measurement information from the second interface portion, and power flow estimation information from the state estimation calculating means with each other to judge reliability of the state estimation calculation in the state estimation calculation means; the control table is configured to be configured by every judgment result of the reliability judging means, a circuit breaker which is to be newly opening-manipulated from a viewpoint of ensuring of power system stability after occurrence of a grid fault in the power system being decided in the control table, wherein the circuit breaker opening command computing means is configured to give a circuit breaker opening command for a circuit breaker by using the information in the control table selected in response to a judgment result of the reliability judging means, wherein a Phase Measuring Unit, PMU, installed in the power system is a phase measuring unit as a time synchronous type sensor using a Global Positioning System, GPS, and the power flow measurement value given by the PMU is a true value, and the reliability judging means is configured to evaluate whether the power flow estimation value of the state estimation calculating unit obtained from the output of a sensor involves a problem in the error or the time synchronization. a first interface portion configured to receive as inputs thereof, for every plurality of circuit breakers, information associated with a voltage and a power flow close to the plurality of circuit breakers concerned, and information associated with a protection relay of the circuit breakers for deciding an opening operation of the plurality of circuit breakers; a second interface portion configured to receive as an input thereof the information associated with a voltage and a power flow containing information associated with measurement time measured in the power system; state estimation calculating means configured to carry out power flow state estimation calculation for every node of the power system by using measurement information, associated with the voltage and the power flow, from the first interface; grid fault judging means configured to judge a grid fault from the information associated with a protection relay from the first interface portion; circuit breaker opening instruction computing means configured to compute and to output a breaker opening instruction by using information in a control table and by using a fault information from the grid fault judging means; characterized by reliability judging means configured to compare power flow measurement information from the second interface portion, and power flow estimation information from the state estimation calculating means with each other to judge reliability of the state estimation calculation in the state estimation calculation means; the control table is configured to be configured by every judgment result of the reliability judging means, a circuit breaker which is to be newly opening-manipulated from a viewpoint of ensuring of power system stability after occurrence of a grid fault in the power system being decided in the control table, wherein the circuit breaker opening command computing means is configured to give a circuit breaker opening command for a circuit breaker by using the information in the control table selected in response to a judgment result of the reliability judging means, wherein a Phase Measuring Unit, PMU, installed in the power system is a phase measuring unit as a time synchronous type sensor using a Global Positioning System, GPS, and the power flow measurement value given by the PMU is a true value, and the reliability judging means is configured to evaluate whether the power flow estimation value of the state estimation calculating unit obtained from the output of a sensor involves a problem in the error or the time synchronization.