An object of the present invention is to provide a simple method of detecting a protein in a liquid, which requires less labor or cost as compared with a conventional method. In the present invention, it has been found that coloring a protein by a color reaction specific to the protein, performing filtration using a filter which easily adsorbs a protein to concentrate the protein on the filter, and checking coloration allows for simply and easily detecting and measuring a protein. In addition, it has been found that filtering a protein using a filter, concentrating the protein on the filter, then coloring the protein by a color reaction specific to the protein, and checking the protein allows simply and easily detecting and measuring a protein.