PURPOSE: To make it possible to reduce the labor of confirmation work of a high water level detector of a water receiving tank by placing a fork member installed to the tip of an expandable rod into contact with a lever of a ball tap and bringing the other end of the rod into contact with a corner part of the water receiving tank.

CONSTITUTION: A fork member 52 of a jig 51 is clamped with a lever 22 of a ball tap 21 while a stopper 54 on the other end of a rod 51 is pressed against a corner part 11 of a water receiving tank 1. Then, the length of the rod 51 is adjusted with a screw member so that the ball 21 may be submerged in the surface of water, thereby fixing a string 55 which prevents the drop of the rod 5 with a manhole 4. At first, the manhole 4 is opened and then a regular position of the ball tap 21 and a water supply stop position are confirmed and a high water level confirmation jig is mounted. In this manner, the ball tap 21 is placed under the surface of water while a control valve 2 is opened, thereby starting the supply of water. Then, the water surface rises and the water comes into contact with an electrode rod 31, thereby making a short- circuit between the electrodes 31 and 32 so that an alarm may sound. Then, the water level which has risen is measured, thereby confirming a regular valve and returning the manhole 4 to an original state and ending the operation.