To detect the gradient precisely even when the length of an air bubble is varied by fluctuation of temperature etc., in an air bubble type water level using the air bubble enclosed with transparent liquid in a tube where a light beam is incident on one side of the tube and received at the other side for detecting the gradient from the variation of the received light amount.

In the water level constituted of the air bubble tube enclosing the transparent liquid and the air bubble, a light projector arranged at one side of the air bubble tube, and light receivers arranged opposite side across the air bubble tube, wherein a pair of light receivers are arranged at both the ends of the bubble at the time the water level is horizontal, and the gradient of the water level can be detected by variation of the light received amount detected by the pair of receivers at both the ends of the air bubble.