1. An output parameter estimation apparatus for estimating an output parameter value from an input data set that is composed of a plurality of input parameter values, wherein the input data set is obtained whenever time series input data is sampled, the output parameter estimation apparatus comprising: field division information storage means for storing field division information showing how an n-dimensional space is divided into a plurality of fields, "n" being an integer no less than 1, and the n-dimensional space having n axes that correspond to n input parameters; field determination means for determining a field, among the plurality of fields, to which the input data set belongs, according to the field division information; fuzzy operation means for calculating a fitness degree of the input data set in each of the determined field and a field adjacent to the determined field by using a fuzzy inference rule for n input parameter values, among the plurality of input parameter values, that correspond to the n input parameters in the n-dimensional space; a neural network, including an input layer, an output layer, and at least one hidden layer between the input layer and the output layer, the input layer, the hidden layer, and the output layer being connected according to a connection condition corresponding to each of the plurality of fields, the connection conditions having been set as a result of learning performed using learning data; route selection means for selecting routes between an introduction layer for the neural network and the input layer of the neural network, based on the fitness degrees obtained by the fuzzy operation means; and input data introduction means for inputting, after the routes are selected by the route selection means, the input data set into the input layer of the neural network via the introduction layer and the selected routes, wherein, after being inputted in the input layer, the input data set is transferred to the output layer via the hidden layer in the connection condition corresponding to the determined field, and as a result the estimated output parameter value is outputted from the output layer.