To extract the position of a bubble in a bubble tube as an electric signal to the outside part.

In a bubble tube sensor, a bubble tube 1 smoothly bent like an arch so that the top part can be formed of the central part is filled with fluid 4 while air bubble 5 is left, and a non-translucent small sphere 6 whose specific gravity is smaller than that of the fluid 4 is allowed to float and held in the air bubble 5. This bubble tube sensor is provided with a position detecting means (photoelectric switch 9 constituted of a projecting element 7 and a light receiving element 8) for detecting the position of the small sphere 6 in a non- contact state, and for outputting it as an electric signal 13 to the outside part.