A mobile object detecting apparatus includes first radiation detecting means; and second radiation detecting means for radiating an electromagnetic wave having the same frequency as the electromagnetic wave radiated by the first radiation detecting means such that the radiated electromagnetic wave passes near a point in the first radiation detecting means from which the electromagnetic wave is radiated, and detecting a standing wave which is generated due to reflection of the radiated electromagnetic wave at an object; wherein a distance, over which the electromagnetic wave radiated by the first radiation detecting means travels until it reaches near the first radiation detecting means, corresponds to a distance of an integral multiple of a wave length of a half cycle of the electromagnetic waves radiated by the radiation detecting means plus a wave length of a predetermined period which is smaller than the half cycle.