To provide an ultrasonic liquid level indicator which is hard to produce errors due to a board thickness of the bottom part of a container, when a liquid level is detected by an ultrasonic transducer suitable for the bottom part of the container.

When an ultrasonic wave is emitted toward the liquid surface through the bottom part of the container 30 by performing a burst drive of the ultrasonic transducer 2 while changing its frequency, extremes appear periodically in response to the appearance of board-thickness inherent frequencies different in order, in detection information by each frequency generated in the ultrasonic transducer 2 following the drive. By using the extremes, therefore, a plurality of board-thickness inherent frequencies different in order of the bottom part of the container 30 are predetermined. By making correction by the use of the board thickness t, on the occasion of calculating the liquid level in the container 30, based on a received signal of a reflected wave RW by the liquid surface with a liquid-level detecting drive frequency, the liquid level is predetermined always accurately, even when the board thickness of the container 30 varies, and, for example, reliability on the occasion of measuring a remaining gas amount in the container 30 from the liquid level is improved.