PURPOSE: To detect a liquid level with a high accuracy by providing an ultrasonic pulse, on a plug body fitted water-tightly to a sensor fitting member provided on the bottom wall surface of a tank containing a liquid.

CONSTITUTION: In a through-hole (g) made in an important point of the bottom wall surface of a tank (t) containing a liquid 1, an ultrasonic level sensor PS is installed to a sensor fitting member (s) consisting of a female plug inserted and provided water-tightly in advance. A plug 1 is a plug body consisting of a male plug fitted water-tightly by pressing into the member (s), and has a recessed part 1a in the center of the inside end face projecting into the tank (t), and a pair of vertical holes 1b, 1b' passing through to the outside end face of the plug body 1, respectively. Also, a connected part of conductors 26a, 26b is coated with a water-proof paint 27. According to said constitution, even in case when a liquid level is ruffled, the liquid level can be detected with a high accuracy.