Photo induced phase separation of a mixture of water and 2-butoxyethanol, in which spiropyran was dissolved as a photoresponsive molecule, was investigated. It was found that the phase separation temperature of the merocyanine (MC) form solution was higher than that of the spiropyrane (SP) form; therefore phase separation was induced by visible light irradiation to the solution which caused the photoisomerization from the MC form to the SP form. The system also exhibits reversible photoinduced phase mixing by irradiation of UV light. The photo-chemical phase separation was also induced by the nanosecond laser pulse irradiation and the dynamics of the phase domain growth were studied.