A fiber sensor for measuring temperature or strain comprises a first polarization-maintaining fiber (12) and a second polarization-maintaining fiber (16). In case of a temperature fiber sensor, the first and second fibers are selected to be unequal in change in phase difference caused by change in temperature but equal in change in phase difference caused by strain. In case of a strain fiber sensor, the first (12) and second (16 ) fibers are selected to be unequal in change in phase difference caused by strain but equal in change in phase difference caused by change in temperature. Axes of polarization of the first (12) and second (16) fibers ar shifted by 90° at a junction of the fibers. With these arrangement, the temperature fiber sensor can measure the change in temperature with high accuracy without being affected by strain, while the strain fiber sensor can measure the strain with high accuracy without being affected by the change in temperature.