To provide a new water level difference measuring apparatus using an optical fiber, making it possible to compare water level differences at two points easily and advantageously in terms of system construction.

Water is introduced into first and second water conveyance chambers 1 and 2 from a portion where a difference in water level occurs. A bellows 3 which is contracted upwards by water pressure is provided in each of the two water conveyance chambers 1 and 2, and the displacement of the end wall 3a of the bellows is transmitted to a balance 5 via a rod 6 connected to the bellows, so that the balance 5 is tilted when a difference in water level occurs. This tilt switches the actuated states of two sensors 7. The sensor 7 causes light transmission losses by pressurizing and deforming the optical fiber by means of a pressurizing element 7a. By checking the operating condition of the sensors by means of a light transmission loss distribution measuring device, the occurrence of the difference in water level can be detected.