Estimating operational states of a plurality of electrical devices connected to an electrical power line, that is, an electricity usage condition is appropriately carried out. Current and voltage are calculated as time-series from the electrical power line to which the plurality of electrical devices is connected. Signal components of power frequency and harmonics thereof are extracted from the time-series of the current and the voltage. Admittance vector time-series are calculated from the signal components. The admittance vector time-series are compared with combinations of feature vectors of the plurality of electrical devices connected to the electrical power line, to obtain an estimated result of the operational states of the plurality of electrical devices.
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