PURPOSE: To provide the sensor in which the detection sensitive to liquid is high and detection speed is high and the winding of a restraining member is easy.

CONSTITUTION: On the peripheral part of a swelling member 15 constituting an optical fiber unit 12, an restraining member 13 is spirally wound with a fixed pitch. The unit 12 is provided with a sensor body 16 comprising an optical fiber 17, and the swelling member 15 covering the body 16. The swelling member 15 is made of a material expanding when absorbing a liquid. The sectional form of the arresting member 13 is flat, and the member 13 is wound so that the even surface 20 along the longitudinal direction of the section is opposed to the peripheral surface of the fiber unit 12. The secondary moment of section of the restraining member 13 is put a quarter or less of the secondary moment of a circle section having the same diameter as the size in the width direction of the section of the restraining member 13.