An object detecting device is provided whereby the number and size of detected objects can be determined without making the construction of the device more complex and the cost of components higher. The object detecting device includes multiple transmitter-receivers for transmitting radio waves and receiving reflected waves, each of the transmitter-receivers receiving both a reflected wave of a radio wave transmitted by itself and reflected waves of radio waves transmitted by other transmitter-receivers, a path-by-path reception distance computing unit for computing the path-by-path reception distance of every reflected wave and an object determination element for determining the number and/or size of detected objects based on the relationship between the reception distances computed by the path-by-path reception distance computing unit.