To establish a convenient procedure for analyzing protein soil, we investigated a method for visually evaluating proteins using a bicinchoninic acid assay performed on a solid surface (filter paper). On the filter paper, the color change proceeded in the assay, allowing detection through measurement of surface reflectivity at 560 nm. The assay was performed over ranges of 4℃ to 37℃ and 0 to 100 min to determine optimal assay conditions, which, for a 30 μ L sample, were a wavelength for measuring surface reflectivity of 560 nm, a time of 30 min, and a temperature of 25℃. The standard curve showed good linearity at y＝－13.65 logx＋64.05 (R²＝ 0.976) over an albumin concentration range of 7.5 to 600 μ g. Color differences were observed over an albumin concentration range of 0 to 300 μ g. Since the color change proceeded at other temperatures and durations, the assay can be conducted under conditions suited to the environment and purpose.