The present invention provides a technique for crystallizing a desired protein at a high probability; namely, a protein crystallizing agent and a method of crystallizing protein. The present invention also provides a technique for determining the conditions for protein crystallization easily with high efficiency; namely, a method of screening the conditions for protein crystallization and a protein crystallization screening reagent. As the protein crystallizing agent, at least one compound selected from the group consisting of a basic amino acid, acidic amino acid, ester derivative of amino acid and amide derivative of amino acid is used, or at least one of these compounds is used in combination with another protein crystallizing agent.