A blood-viscosity reducing agent contains a protein derived from Bacillus subtilis natto and including, sequentially from an amino group terminal, a first structural amino acid sequence having alanine, threonine, aspartic acid, glycine, valine, glutamic acid, tryptophan, asparagine, valine, aspartic acid, glutamine, isoleucine, aspartic acid, alanine, proline, lysine, alanine, tryptophan, alanine, leucine, glycine, tyrosine aspartic, acid, glycine, threonine, glycine, threonine, valine, valine, alanine, serine, isoleucine, aspartic acid, threonine, glycine, valine, glutamic acid, tryptophan, asparagine, histidine, proline, alanine, leucine, lysine, glutamic acid, lysine, tyrosine, arginine, glycine, tyrosine, asparagine, proline, glutamic acid, asparagine, proline, asparagine, glutamic acid, proline, glutamic acid, asparagine, glutamic acid, methionine, asparagine, tryptophan, tyrosine, aspartic acid, alanine, valine, alanine, glycine, glutamic acid, alanine, serine, proline, tyrosine, aspartic acid, aspartic acid, leucine, alanine, histidine, glycine, threonine, histidine, valine, and threonine, a second structural amino acid sequence having alanine, phenylalanine, serine, glutamic acid, aspartic acid, glycine, glycine, threonine, aspartic acid, alanine, aspartic acid, isoleucine, leucine, glutamic acid, alanine, glycine, glutamic acid, tryptophan, valine, and leucine, a third structural amino acid sequence having aspartic acid, alanine, glutamic acid, glycine, asparagine, proline, histidine, proline, glutamic acid, methionine, alanine, proline, aspartic acid, and valine, and a fourth structural amino acid sequence having valine, proline, glycine, glutamine, alanine, tyrosine, glutamic acid, aspartic acid, glycine, tryptophan, and aspartic acid, thereby reducing the viscosity of whole blood.