Comparison indices each for two components incorporated in a source program are used for easy and quantitative evaluation of functional redundancy in the program, effective and accurate extraction of redundant code segments from the program and also effective and accurate extraction of components to be modified simultaneously. A tree T is entered and an initial level of functional redundancy m(P) is set at 0 in a program P expressed by the tree T. The top node of the tree T is selected as a node N. A specific computation is performed for the top node selected as the node N with attribute information including the similarity and the number of children of the node N to obtain a level .alpha.. The level .alpha. is added to the functional redundancy m(P). The specific computation is performed for every node in the tree T, to obtain functional redundancy m(P) including the total of .alpha. for all nodes.