Refining method

A refining method for crystallizing a compound with a plurality of crystal forms, comprising:

• setting, as a target wavelength, a specific infrared wavelength at which a specific crystal form of the plurality of crystal forms precipitates from a solution of the compound dissolved in a solvent;
• and using an infrared radiation apparatus capable of emitting infrared radiation including the target wavelength to evaporate the solvent and precipitate the specific crystal form while irradiating the solution with infrared radiation including the target wavelength,
• wherein the specific infrared wavelength is set as the target wavelength based on an infrared absorption spectrum of the crystal form and a dissolution rate of the compound in the solvent, and
• wherein the dissolution rate is a time to dissolve the compound in the solvent, which is determined before performing the refining method.