PURPOSE: To vary the sectional form of one waveguide without applying a force to the overall cross section of the waveguide and to ensure that the pressure is detected by providing a waveguide forming member with a plurality of waveguides each of which extends along the length of the waveguide forming member.

CONSTITUTION: A pressure sensor 1 is made from a material that can be elastically deformed, such as natural rubber, and has a long waveguide forming member 2 having a flat, crescent-shaped cross section. The member 2 comprises an outside wall portion 21 of circular cross section and an inside wall portion 22 of circular cross section. The internal space of a hollow body is divided along the length of the member 2 into two parts by a bulkhead portion 23 formed integrally with the wall portions 21 and 22, and two waveguides 6a, 6b are formed in the member 2 along the length of the member 2. A transmitter 3 and a receiver 4 are mounted to one end of the member 2 to correspond to the first and second waveguides 6a, 6b, respectively, and a wall end portion 25 for blocking the other end is formed. A U-shaped connecting pipe 26 is mounted on an end wall portion 25.