1. A software maintenance supporting device comprising 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 analysis means includes variable dependence relation analysis means for recognizing another variable to be referred to when calculating the variable, and extracting variable dependence relation data, 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.