1. A plant operation support system in which either of a plurality of operation control variables and operation state evaluation variables of a plant are calculated through a multilayered neural network based on data indicating a plant state, and the calculated results are communicated to an operator of said plant to support plant operation, said support system comprising: means for analyzing an internal causality between neurons in said neural network; means for displaying quantitative guidance associated with said neural network and also an analyzed result of the internal causality between the neurons as an association reason; data selection means for determining the magnitude of causality between input and output of said neural network and selecting data which has a causality larger than a preset reference; and data display means for displaying the selected data, wherein said data selection means selects such a data item that an absolute value of said value resulting by summing a product between the connection intensity in a path reaching an output layer neuron of interest and a differential value of a conversion function used at a passing neuron for all the paths via hidden layer neurons exceeds a predetermined value, as data having the large causality.