A method for producing a lactam by using an organic solvent as a substrate solution, which makes it possible to introduce a high-concentration oxime into flowing high-temperature and high-pressure water, thereby allowing the high-concentration lactam to be synthesized with a high efficiency. With this method, the lactam is continuously synthesized at a high rate from the oxime under high-temperature and high-pressure water mixture conditions within a temperature range of no less than 250° C. and a pressure range of no less than 15 MPa.