1. A system having a plurality of devices and a controller for controlling operations of said plurality of devices; wherein said controller is arranged to control said plurality of devices according to a plurality of rules corresponding to inter-relationships of data items relating to the operations of said plurality of devices expressed by a neural network model having a hierarchical structure constructed of an input layer, at least one hidden layer and an output layer, said data items including output values as control variables to at least one of said plurality of devices and input values as state variables of at least a further one of said plurality of devices affecting on said output values, said controller comprising: means for analyzing said data items to make evaluation indices obtained from weighing factors to inner combinations among output layer neurons, hidden layer neurons and input layer neurons of said neural network model; means for calculating a summation of absolute values of said evaluation indices;. means for normalizing each of said evaluation indices by said summation to obtain relative evaluation indices; means for selecting pairs of said input and output values as to which of said relative evaluation indices satisfy a predetermined relationship with a predetermined threshold value; means for deriving said plurality of rules corresponding to said inter-relationships between said selected input and output values; and means for controlling said plurality of devices based on said derived rules.