A surface-treated steel plate obtained by forming, on at least one surface of the steel plate by cathodic electrolysis, a surface-treating film which contains a mixed oxide of an oxide of Fe and at least one of an oxide of Zr or an oxide of Ti, and a polycarboxylic acid, the amount of Zr or Ti, or the total amount of Zr and Ti if both of them are present, being from 3 to 300 mg/m2 and the amount of C being from 0.1 to 5.0 mg/m2.