PURPOSE: To generate no initial error by constituting so that a liquid surface in a lower pot does not reach an opening part in a lower pot of an air pipe, even when a liquid stabilizer has been inclined to the maximum.

CONSTITUTION: The inner capacity of a pot 1 is made large, a liquid stabilizer is balanced, and in case when a liquid 7 of an equal quantity exists in the pots 1 and 1', for instance, a liquid surface of the liquid 7 in the pot 1 is positioned in a far lower part than the inside surface of a cover 6, namely, an opening end 5' of an air pipe 5. Accordingly, even when the liquid stabilizer is upset, the liquid surface of the liquid 7 in the pot 1 stays sufficiently in the pot 1 as shown by a dotted line 13, and the liquid surface does not reach the opening end 5' absolutely.