A method for easily producing high-purity polydimethylsilane or polydiphenylsilane, where by-products such as alkali metal salt and alkaline earth metal salt can be efficiently removed, is provided. Dimethyldichlorosilane or diphenyldichlorosilane is reacted with an alkali metal such as metal sodium and metal magnesium and/or an alkaline earth metal in an organic solvent such as toluene to obtain crude polydimethylsilane or crude polydiphenylsilane, methanol having dissolved therein an ether ester-type nonionic surfactant or a surfactant such as alkylbenzenesulfonate is added to the crude polydimethylsilane or the crude polydiphenylsilane to deactivate the remaining alkali metal and alkaline earth metal, and the crude polydimethylsilane or the crude polydiphenylsilane is washed with water in the presence of a surfactant to efficiently remove an alkali metal salt, an alkaline earth metal salt and the like, whereby high-purity polydimethylsilane or polydiphenylsilane is obtained.