Provided are a liquid crystal alignment film capable of being given anisotropy through photoalignment and stable to light, and a liquid crystal display device capable of keeping a high voltage holding ratio without degradation of display quality even when exposed to strong light for a long period of time. To provide these, used is a liquid crystal aligning agent for photoalignment which contains a polymer having a structural unit containing a photoreactive structure and in which the structural unit containing a photoreactive structure undergoes chemical reaction by heating.