An object detection system includes a depth image detector and a moving object extractor. The depth image detector detects a depth image from an external environment. The moving object extractor extracts a moving object desired to be extracted from the depth image. The moving object extractor registers in advance the depth image in a memory as a background while the moving object to be extracted does not exist, and extracts only a pixel whose current depth is present on a nearer side than a depth of the background as a candidate for a pixel corresponding to the moving object to be extracted.