1. A program development supporting apparatus that groups a plurality of events each executed in an information processor to divide the events into a plurality of parallel execution units to be executed in parallel with each other, said apparatus including at least one computer comprising: a directional graph acquisition section that acquires directional graph data expressing each of the plurality of events as a vertex and a restriction on the execution order between two of the plurality of events as a directional branch from a model of the program, wherein the model is separate from processing content for each of the plurality of events for the program; an inverse chain partial set extraction section that traces the directional branch from each event in the forward direction to extract from the directional graph data an inverse partial set that is a combination of the events having such a relationship that any one of the events cannot be reached from the other events; and a parallel execution unit assignment section that assigns the plurality of events belonging to the inverse partial set to units different from each other in the parallel execution units.