PURPOSE: To stably measure overrun without obstructing the flow of gas- containing food in a feed pipe by measuring the attenuation of the sound from the sound source supported on the feed pipe.

CONSTITUTION: An overrun measuring device 10 constituted of respective members such as a tuning fork 11 or the like is received in a sound-insulating box 17 and the leg part 15 of the tuning fork 11 is fixed to the upper part of feed piping 9a. When the tuning fork 11 is struck by a hammer 12, sound propagates through the piping 9a from the leg part 15 and rosonates within the piping 9a to gradually attenuate. This sound is collected by a highly directional microphone 14 and the timewise change of sound pressure is stored in a control part as attenuation data and an attenuation time is determined on the basis of the attenuation data. Since the sound propagating through the piping 9a is attenuated corresponding to the ratio of air bubbles in the liquid within the piping 9a, if the attenuation time is determined, the overrun value (content of air bubbles) in the liquid can be calculated.