PURPOSE:To eliminate light propagation loss and to obtain a hydrophone device scarcely being influenced by noise by adhering tightly an optical fiber deforming element to a photodetecting element. CONSTITUTION:When light suppied from a light emitting source 24 to the core of the optical fiber 23 is passed through a part of the optical fiber 23 which is sandwiched betweeen the deforming elements 21A and 21B, the light transmitted from the core to a clad layer in accordance with the curvature of the bent optical fiber 23 and than leaked from the clad layer to the outside of the optical fiber 23. The light leaked from the clad layer to the outside is transmitted to the elements 21A, 21B through the projected parts of the elements 21A, 21B, reached to the photodetecting eleents 22A, 22B tightly adhered to the elements 21A, 21B and converted into an electric signal by the elements 22A, 22B and the electric signals are supplied to a detector 25 to that the electric signal corresponding to the deformed variable of the optical fiber 23 by the elements 21A, 21B is outputted or displayed. Thus, the light leaked from the clad layer of the optical fiber to the outside cna be also detected.
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