1. A non-transitory computer-readable recording medium having a program stored therein for causing a computer to execute a process of determining an installation order of a plurality of update programs corresponding to a plurality of respective pieces of software, the installation being executed for updating current versions of the plurality of pieces of software to newer versions, respectively, the current versions having been installed on a computer to be updated, the process comprising: applying a function outputting an index representing a degree of likelihood of existence of a version of a piece of software on the computer, to information about each one of the pieces of software existing on a plurality of known computers, to calculate the index for each combination of the current versions and the newer versions of the plurality of pieces of software; and searching for the installation order satisfying a predetermined condition based on a total of the indices corresponding to the combinations of the current versions and the newer versions possibly realized when installing the update programs corresponding to the plurality of pieces of software one by one from a state of all the current versions to a state of all the new versions.