Disclosed is a polyorganosiloxane which has a repeating unit represented by the general formula (1) and has a weight average molecular weight of 1,000 to 100,000: (1) wherein Z represents a photosensitive group capable of causing a crosslinking reaction or an isomerization reaction when irradiated with light having a wavelength of 200 to 400 nm; and X represents a hydroxy group, an alkyl group having 1 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms or an alkoxyl group having 1 to 20 carbon atoms, or may be crosslinked with X in other repeating unit through -O-. The polyorganosiloxane has excellent coatability and can be used for the liquid crystal alignment film formation which can impart a liquid crystal alignment capability through the irradiation with a polarized or non-polarized radioactive ray without the need of conducting any rubbing processing.