1. A video image monitoring system comprising: a video-image-capturing section configured to put out image data based on a video image signal obtained by using an image-capturing device; a mobile-object-candidate-area-detecting section configured to extract a candidate area of a mobile object from the image data; and a mobile-object-detecting section configured to determine whether the candidate area is the mobile object, wherein the mobile-object-candidate-area-detecting section comprises: a direction code calculation unit configured to calculate a direction code obtained by quantizing a brightness gradient direction of the image data; a spatio-temporal histogram calculation unit configured to calculate a spatio-temporal histogram which represents a frequency of the direction code appearing in a predetermined spatio-temporal space; and a spatio-temporal space evaluation value calculation unit configured to calculate a statistical spatio-temporal space evaluation value of the spatio-temporal histogram, and wherein the mobile-object-detecting section is configured to determine whether the candidate area is the mobile object based on the spatio-temporal space evaluation value.