In the present invention, from a smart meter installed at a plurality of consumer locations, active power is acquired at specific time intervals (S14), and from a sensor installed on the upstream side of the plurality of consumers, the value of passing reactive power and of voltage are acquired at specific time intervals (S12). A relational expression for finding an unknown parameter (reactive power) from the value of active power is determined (S16), and the probability of the relational expression (for example, the distribution of kj) is found using the effective power and the value of the reactive power and of the voltage (S18-S22). Using the probability of the relational expression, the constraint conditions of the unknown parameter are determined (S24), and the unknown parameter is estimated by optimization processing in which the constraint conditions are used (S26). Due to this configuration, an unknown parameter can be estimated regardless of the number of unknown parameters.
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