PURPOSE: To obtain a high-purity biopterin, easily, without using a column, by preparing an alkali metal salt of biopterin from an alkaline aqueous solution of crude biopterin by a specific method, dissolving the salt in water, and treating the solution with an acidic aqueous solution.

CONSTITUTION: 1 equivalent of crude biopterin (D-erythro, D-threo, L-erythro, L- threo compounds or their mixture) is dissolved in 1W3 equivalents of an aqueous solution preferably containing KOH. The concentration of the obtained biopterin solution is 10W50W/V%. A hydrophilic organic solvent (e.g. ethanol) is added to the solution at 20W80°C to precipitate the alkali metal salt of biopterin.. The precipitate is separated by filtration, washed with methanol, and dried in vacuum. The obtained alkali metal salt of biopterin is dissolved in water and treated with an aqueous solution of a water-soluble organic acid (e.g. formic acid). The precipitated crystal is separated by filtration, washed with methanol, and dried in vacuum to obtain the objective product.