PURPOSE: To obtain the title compound useful as an organic solvent industrially and advantageously while readily and inexpensively removing a by-product by bubbling an oxidizing gas in a thiophenol-containing N-methyl-2-pyrrolidone and distilling the solution.

CONSTITUTION: An oxidizing gas is bubbled in a thiophenol-containing N- methyl-2-pyrrolidone and the solution is distilled to give purified N-methyl-2- pyrrolidone. The gas used for bubbling is utilized as an oxidizing agent and a mixed gas of oxygen, ozone, etc., and an inert gas is preferable. Air is especially preferable as the gas. The bubbling is carried out at 10-150°C, preferably 30-120°C for 1-15 hours.