PURPOSE: To detect the pressures at two places by one piece of a sensor and to measure these pressure difference by impressing external pressure from two directions which are at right angles each other and by transmitting a polarized light through two pieces of photoelastic element that the optical path is arranged on the same axis.

CONSTITUTION: When the first pressure is applied on a diaphragm 6a from the external part the same pressure is impressed on the first photoelastic element 5a. When the second pressure is applied on the second diaphragm 6b of the opening part of the side face of a case 12 the same pressure is impressed on the second photoelastic element 5b. The first pressure is impressed on the element 5a, then a phase difference is caused with the double refraction of the linear polarized light being made incident, making a plane polarized light. When this plane polarized light is made incident on the element 5b on which the second pressure is impressed, the double refraction is again caused, making the plane polarized light having the phase difference corresponding to the double refraction. Namely the light at the outlet port of the element 5b becomes two plane polarized light rays having the characteristics of the double refraction and phase difference corresponding to the difference of the first and second pressures. It is converted into electrical signal at the inside of a light receiver 11.