Disclosed herein is a precursor solution for forming a biaxially oriented buffer layer through low-temperature heat treatment, by which a highly oriented buffer layer can be formed even when the precursor solution is heat-treated at a low temperature of 1000° C. or lower at the time of forming a buffer layer through a wet chemical method. The precursor solution is prepared by adding a carboxylate or an alkoxide of bismuth, boron, lead, gallium, or the like, which is a metal salt for forming an oxide having a low melting point of 1200° C. or lower after pyrolysis in an oxygen atmosphere, to a precursor solution for forming a buffer layer through a wet chemical method.