In a method for separating isophthalonitrile from a gas produced by causing m-xylene to react with ammonia and oxygen-containing gas in the presence of a catalyst, the gas is brought into contact with an organic solvent having a boiling point lower than that of isophthalonitrile; a liquid in which isophthalonitrile is trapped in a trapping step is distilled, to thereby recover isophthalonitrile and the organic solvent from the top of the column and separate at the bottom of the column impurities having boiling points higher than that of isophthalonitrile; and the organic solvent is recovered from the top of the rectification column and liquefied isophthalonitrile of high purity is recovered at the bottom of the column. Thus, loss of isophthalonitrile and plugging of a vacuum-evacuation system caused by isophthalonitrile migrating from a condensation system during distillation under reduced pressure can be prevented, and high-purity isophthalonitrile can be produced at high yield constantly for a long period of time.