Abstract: A method and an apparatus for determining a length of a pipe utilizing sound waves, which comprise: transmitting sound waves having a prescribed frequency and a prescribed transmission intensity toward the interior of a pipe at one end thereof by an oscillator; receiving the thus transmitted sound waves at the other end of the pipe by a receiver; measuring a propagation time required for the sound waves to pass through the air in the pipe from the one end to the other end thereof by a clock; and determining a length of the pipe by a length calculator on the basis of a predetermined propagation velocity of the sound waves in the air in the pipe and the thus measured propagation time of the sound waves.