1. A device for detecting obstacles, for use in vehicles comprising: a plurality of sensor means for emitting beams toward the surrounding of the vehicle and for producing reception signals depending upon the intensities of the reflection beams; and a signal processing means for detecting obstacles around said vehicle based upon changes in the reception signals with the passage of time; wherein said signal processing means includes: detection region-setting means for dividing the region around said vehicle into a plurality of matrix-like detection regions; probability-of-presence operation means for operating the probabilities of presence of said obstacles in said detection regions as in-the-region presence probabilities; a probability distribution operation means for operating the distribution of presence probabilities by synthesizing said in-the-region presence probabilities for each of the detection regions; and an obstacle detection means for detecting the positions and sizes of said obstacles from said regions where said obstacles exist by judging the regions where said obstacles exist based on said distribution of presence probabilities; wherein said obstacle detection means judges detection regions exhibiting the in-the-region presence probabilities that are larger than a predetermined threshold value to be the regions where said obstacles exist among the in-the-region presence probabilities in the detection regions in said distribution of presence probabilities.