PURPOSE: To obtain a method for detecting the pressure of fluid to be detected from the loss distribution of an optical fiber by using OTDR in which the pressure variation due to temperature variation of the fluid can be detected separately from the pressure variation due to abnormality such as leakage.

CONSTITUTION: An optical fiber 2 connected with an OTDR is provided with a sensor. The sensor comprises a bellows for reference fluid, a bellows 5 disposed oppositely to the bellows 6 and expands/contract with the differential, pressure between a fluid A to be detected and a reference fluid B, and a part 7 for imparting stress to the optical fiber 2 through expansion/contraction of the bellows 5, 6. When the fluid A is 'leaked', the part 7 is driven with the differential pressure to increase transmission loss of the optical fiber 2. The OTDR detects increase of the transmission loss.