A tracking system of the present invention includes a transponder 20 attached on a moving object 2 and an autonomous mobile unit 1 for tracking the transponder 20. The autonomous mobile unit 1 has an environment detecting means for acquiring reflection environment information relating to a wall surface 6a, 6b and 6c existing in the vicinity. In this system, the following tracking cycle is executed. The autonomous mobile unit 1 transmits a first ultrasonic wave, and upon receipt of it, the transponder 20 transmits a second ultrasonic wave. By receiving the second ultrasonic wave, the autonomous mobile unit 1 acquires reception information relating to a direct wave that directly arrives at the autonomous mobile unit 1 from the transponder 20 and a reflected wave that arrives via the wall surface. Using the reception information and the reflection environment information, the autonomous mobile unit 1 decides whether the received second ultrasonic wave is the direct wave or the reflected wave and processes the reception information based on the decision to estimate and track the position of the transponder 20.