PROBLEM TO BE SOLVED: To provide a coating liquid for boron and aluminum diffusion which: allows the density of a boron compound to be controlled to a desired one while using a propyleneglycol-based solvent as a solvent of a coating liquid for impurity diffusion; contains an aluminum compound used as a second impurity; enables boron and aluminum to be uniformly diffused into a semiconductor silicon substrate; causes neither variation in the surface specific resistance of the semiconductor silicon substrate after the diffusion, nor impurity crystalline precipitation; and allows the boron and aluminum to be diffused at desired densities.SOLUTION: The coating liquid for diffusion for forming a P-type diffusion layer in a semiconductor silicon substrate comprises: (a)a boron compound and (b)an aluminum compound which are used as materials for forming a P-type diffusion layer in a silicon substrate; a solvent including (c)a propyleneglycol derivative and (d)water; and a coating film forming agent consisting of (e)a polyvinyl alcohol-based resin having a saponification degree of 96 mol% or less. The content of sodium in (e) the polyvinyl alcohol-based resin is 0.2 mass% or less to the mass of the polyvinyl alcohol-based resin. The coating liquid for diffusion is arranged for the purpose of diffusing boron and aluminum into a semiconductor silicon substrate.