A rapid and easy immunochromatography assay using dye-labeled cellulose nanobeads (CNBs) was developed to detect proteins with hexa-histidine tag (His-tag) to characterize recombinant proteins during purification. Recombinant ATG8 protein was used as a His-tagged protein, and ATG8-conjugated CNBs (A-CNBs) were prepared. The original ATG8 in the sample solution competed with A-CNBs for anti-His-tag antibodies spotted on to the strip resulting in an inverse relationship between ATG8 concentration and the colorimetric signal. The usefulness of this method was shown by adding ATG8 to a 1 % Escherichia coli extract. In addition, this assay can be used to detect other His-tagged proteins without protein-specific antibodies. Because the identification of fractions containing His-tagged proteins by western blotting or ELISA is labor-intensive and expensive, our method provides an efficient and cheaper alternative.