1. An object detection apparatus comprising: a posterior probability calculation portion utilizing an occurrence probability of a background and a foreground acquired by utilizing a characteristic quantity extracted from each pixel of an input image and a probability density function, a posterior probability of the background and the foreground for the previous input image to the input image, and a conditional probability indicating a relation of an event (background or foreground) to the vicinity of an attentive pixel in a spatial direction and a relation of an event to the vicinity of the attentive pixel in a temporal direction so as to calculate a posterior probability of the background and the foreground from a probability model set up based on a tendency that the same event appears in the vicinity of the attentive pixel in the spatial and temporal directions; and an object determination portion for determining an object from comparison of the posterior probability of the background with that of the foreground.