PURPOSE: To realize a compact and light-weight converter, by constituting a converter between the pressure and the optical signal phase by means of a single mode optical fiber and a means to apply the pressure to a part of the optical fiber.

CONSTITUTION: The optical fiber-pressure-phase converters 18 and 18' consist of a single mode optical fiber 24 with a protective coat that is used as an optical signal transmission line plus a means 26 that applies the pressure to a part 24b of the fiber 24 where the protective coat 24a is torn off. This pressure applying means includes a U-shaped frame body 28, glass plates 30 and 30' holding the part 24b between them, and a pressure applying element 34 which applies the prescribed load to the part 24b by means of a screw 32 screwed into the body 28. When the load is applied by the screw 32 to the area between the plates 30 and 30' from the element 34. As a result, the section of the part 24b has the distortion to induce the property to be bent. The optical signal passing through the strained section is polarized to have a shift of phase. In such a way, a compact and light-weight structure is attained for a converter.