PURPOSE: To improve the safety and the reliability of a pressure detector regardless of various conditions of applied place thereof by employing a pressure sensitive element elastically deforming in response to the external pressure in a pressure sensing unit and varying the incident light amount to a photodetector by the pressure sensitive element.

CONSTITUTION: A light ray a irradiated from a light emitting element 1 is branched by a half mirror 2 into reflected light ray a1 and the transmitted light ray a2 to an optical fiber 4. The light ray a2 is enlarged by a lens 5 attached to one end of the fiber 4, and further irradiated as a light ray a3 to a diaphragm 6. The reflected light amount is varied thereat in response to the curved deformation of the diaphragm 6 responsive to the external pressure P by the liquid or gas. The light ray a4 is again passed through the lens 5 and the fiber 4, branched by the half mirror 2, incident as a light ray a5 to the photodetector 3, where it is converted an electric amount. Thus, an electric signal from the photodetector 3 enables high reliability and safety of detecting the extent of the pressure P.