1. A software maintenance supporting device comprising a hardware processor and source code analysis means for analyzing a source code of software loaded in an electronic control device to control a control target, and outputting analysis information related to a variable in the source code, wherein the source code comprises a plurality of functions, the source code analysis means includes variable dependence relation analysis means for analyzing a relationship of influence between variables that are included in different ones of the functions, and extracting variable dependence relation data based on the analysis, and variable dependence relation network analysis means having variable cluster analysis means for extracting variable cluster data in which a set of all variables is divided into variable clusters including at least one variable, based on the variable dependence relation data, and the device includes variable dependence relation display means for illustrating an inter-variable dependence relation as a directed graph in which variables are expressed as nodes and the dependence relation is expressed as a link, based on the variable dependence relation data, and highlighting variables belonging to the same cluster based on the variable cluster data, wherein the variable dependence relation display means displays a modularity index calculated based on the number of variable dependence relations within the divided clusters and the number of variable dependence relations between the divided clusters, based on the variable cluster data.