1. A vehicle periphery monitoring system that performs monitoring of a vehicle periphery based on an image of the vehicle periphery captured by an onboard camera, comprising: an object behavior analysis unit that determines whether an observation object is a pedestrian relatively approaching the vehicle to which the monitoring system is installed based on a change rate of a size of an image of the observation object between a plurality of captured images captured at different time points by the onboard camera, and presence or absence of deformation of the image of the observation object between the captured images.