1. A driving state monitoring apparatus for a vehicle, for monitoring a driving state of a driver of said vehicle, comprising: behavior parameter detecting means for detecting a behavior parameter indicative of a lateral movement of said vehicle; vehicle speed detecting means for detecting a vehicle speed of said vehicle; behavior reference setting means for setting a behavior reference according to changes in the behavior parameter; running locus calculating means for calculating a running locus of said vehicle according to the behavior parameter, the behavior reference, and the vehicle speed; vehicle behavior feature parameter calculating means for calculating a plurality of vehicle behavior feature parameters indicative of a feature of vehicle behavior according to the running locus; and driving state determining means for determining whether or not the driving state of the driver is proper according to the plurality of vehicle behavior feature parameters.