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Ferroelastic domain boundaries in semiconductor bismuth vanadate, BiVO4, are examined using second harmonic generation (SHG) microscopy. Although the bulk is centrosymmetric, domain boundaries produce homogeneous SH signals. The polarization dependences of SH intensities exhibite strong anisotropy compatible with the polar symmetry m. The present results are compared with the experimental results of other ferroelastics we have observed so far. Unlike other ferroelastic materials, the directions of the SH maxima are in the same direction for all domain boundaries. Domain boundaries in ferroics are known to exhibit physical properties that do not exist in the bulk. Typical examples are domain boundaries with much higher conductivity than the bulk 1-6 , even being superconducting 7 , charged domain boundaries 8-15 , and polar domain boundaries 16-22. These phenomena have been directly measured by state-of-the-art techniques, such as aberration-corrected transmission microscopy, electron holography, atomic force microscopy etc. The fundamental significance of these phenomena lies in the possible development of new technologies where the domain boundary is the main element of the device. Racetrack memories 23 and logics for electronic circuits 24 are prominent examples of applications, and some of them have been already been achieved by using magnetic domain boundaries. Ferroelastic domain boundaries have several advantages for such future devices 25. Since the width of a ferroelastic domain boundary is much thinner than any magnetic boundaries, a high-density memory device can be realized. In particular, the existence of polar properties at a ferroelastic domain boundary is one of the most promising candidates, because the localized polarization

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