A strain sensor includes a marker, detectors and a calculator. The marker is disposed on a surface of a measurement object and includes a strain body and surface plasmon generating particles. In the strain body, a strain is formed by a load. The surface plasmon generating particles are arranged in two directions which are parallel to two in-plane directions of a light receiving surface of the strain body. The first detector detects a spectral intensity of a light which has been reflected on the marker or has passed through the marker. The second detector detects absorption spectral peaks corresponding to the respective array directions of the particles from the spectral intensity. The calculator calculates the quantity of the strain of the marker based on a difference in wavelength of the two absorption spectral peaks.