A parameter estimating device for identifying estimated values of a plurality of parameters representing a state of a predetermined device changing according to operation, the parameter estimating device comprising: a state quantity measurement unit adapted to measure and acquire a plurality of measured values for state quantities having a correlation with the plurality of parameters at every predetermined time; a state quantity calculation unit adapted to acquire initial estimated values and to calculate calculated values corresponding to the plurality of measured values by substituting said initial estimated values given as the plurality of parameters in a correlation expression of the estimated values and the state quantities; and an optimum estimated value identification unit which is adapted to output the initial estimated values to the state quantity calculation unit and to repeatedly obtain, while changing at least one of the estimated values, an error evaluation function value calculated by summing a value obtained by multiplying the total sum of the absolute quantities of the errors of the plurality of calculated values corresponding to the plurality of measured values by a first coefficient and a value obtained by multiplying a value obtained by dividing the standard deviation of the errors by the average value of the errors by a second coefficient and executes estimated value identification processing for identifying an optimum estimated value having a minimum error evaluation function value, characterized in that the optimum estimated value identification unit is adapted to repeatedly execute the estimated value identification processing in which the first coefficient is set relatively greater with respect to the second coefficient, with an optimum estimated value identified by the estimated value identification processing in which the second coefficient is set relatively greater with respect to the first coefficient as an initial value. a state quantity measurement unit adapted to measure and acquire a plurality of measured values for state quantities having a correlation with the plurality of parameters at every predetermined time; a state quantity calculation unit adapted to acquire initial estimated values and to calculate calculated values corresponding to the plurality of measured values by substituting said initial estimated values given as the plurality of parameters in a correlation expression of the estimated values and the state quantities; and an optimum estimated value identification unit which is adapted to output the initial estimated values to the state quantity calculation unit and to repeatedly obtain, while changing at least one of the estimated values, an error evaluation function value calculated by summing a value obtained by multiplying the total sum of the absolute quantities of the errors of the plurality of calculated values corresponding to the plurality of measured values by a first coefficient and a value obtained by multiplying a value obtained by dividing the standard deviation of the errors by the average value of the errors by a second coefficient and executes estimated value identification processing for identifying an optimum estimated value having a minimum error evaluation function value, characterized in that the optimum estimated value identification unit is adapted to repeatedly execute the estimated value identification processing in which the first coefficient is set relatively greater with respect to the second coefficient, with an optimum estimated value identified by the estimated value identification processing in which the second coefficient is set relatively greater with respect to the first coefficient as an initial value.