1. An outside-vehicle monitoring system comprising: image-pickup means for picking up an image of an object outside a vehicle; three-dimensional-object recognition means for recognizing a three-dimensional object outside the vehicle by processing the image picked up and for extracting a prospective pedestrian from the recognized three-dimensional object; temperature-detection means for detecting a temperature of a detection field overlapping a view field of the image-pickup means in a vehicle-width direction; and pedestrian-determination means for determining whether or not the prospective pedestrian is a pedestrian according to fluctuations in a value of the detected temperature in the same direction as position of the prospective pedestrian.