1. A printer driver comprising: a core driver performing a constraining process to a print setting based on function information containing a constraint for the print setting and stored in a storage unit, and being enabled to accept an addition of a plug-in, wherein the core driver expands basic function information corresponding to the core driver onto a memory as information having a tree structure, when plug-in function information corresponding to the plug-in is stored in the storage unit, the core driver expands the plug-in function information on the memory as information having a tree structure, the core driver merges the information expanded from the plug-in function information into the information expanded from the basic function information, and the core driver performs the constraining process to a print setting based on a parent-and-child relation in a tree structure of the information thus merged.