1. A vehicle planning support system for supporting the planning of new vehicles by building vehicle models, comprising a database for storing vehicle data for building vehicle models, and a computing device utilizing the vehicle data stored in the database to build planned vehicle models for the vehicle being planned, wherein performance evaluation values for quantifying performance evaluations of the vehicle with respect to each of a predetermined evaluation items are included in vehicle data stored in the database for a plurality of existing vehicles, and the computing device has a target value setting section for setting target values with respect to each evaluation item based on a distribution of performance evaluation values for at least a section of comparison subject existing vehicles among the existing vehicles, a performance calculation section for computing performance evaluation values for each evaluation item with respect to the planned vehicle models, and a degree-of-completion calculation section for obtaining a target degree-of-completion by comparing target values and performance evaluation values with respect to each evaluation item.