The present invention intends to provide a high sensitivity method for identifying the type of a protein contained in each cell with a diameter of ten to several tens μm and a sample treatment solution required for it. The above is achieved by three steps of (1) treating a slice of a diseased tissue with an aqueous solution containing gold particles and a digestive enzyme, and digesting restrictively a protein of interest, (2) measuring a 2-dimensional distribution of fragmented peptides by TOF-SIMS, and (3) visualizing a 2-dimensional distribution of the target protein in the slice of the diseased tissue using the results of the proteome analysis and by means of a numerical analysis.