1. An obstacle detection system for a vehicle capable of detecting objects as obstacles existing around a vehicle based on a detection signal irradiated toward and reflected from the objects per predetermined detecting period of scanning the detection signal, comprising: an object sensor configured to detect existence/non-existence and a position of an object within a predetermined detection range outside of the vehicle based on the detection signal received; probability distribution determining means for determining an existence probability distribution of the obstacle in a detecting direction toward the obstacle from the object sensor, and a maximum value in the existence probability distribution being used as a maximum probability of position of the obstacle, and the existence probability distribution having a range gradually decreased before and after the maximum probability of position of the obstacle in the detecting direction; primary probability relationship making means for making a primary existence probability relationship representing a primary probability of existence of the obstacle in a horizontal plane coordinate using a plurality of the existence probability distributions throughout the entire of the predetermined detection range; combining means for making a combined existence probability relationship by combining the primary existence probability relationship obtained in a current detecting period and a secondary existence probability relationship estimated based on a previous detection signal in a previous detecting period corresponding to the primary existence probability relationship of the current detecting period; speed calculating means for calculating an estimated moving speed of the obstacle during a period from the detecting period of two times before to the previous detecting period based on the combined existence probability relationship obtained in the previous detecting period and the combined existence probability relationship obtained in the detecting period of two times before; and secondary probability relationship making means for making the secondary existence probability relationship based on the estimated moving speed of the obstacle and the combined existence probability relationship during the previous detecting period.