The present invention provides a method for producing N-phosphonomethylglycine which comprises reacting an aminomethylphosphonic acid with glycolonitrile, or formaldehyde and hydrogen cyanide in situ, under an alkaline condition to convert the aminomethylphosphonic acid into an N-phosphonomethylglycinonitrile salt or a mixture of an N-phosphonomethylglycinonitrile salt and N-phosphonomethylglycinonitrile, and then hydrolyzing the product under an acidic condition. Subsequent to a reaction step for the production of N-phosphonomethylglycinonitrile, the reaction product is hydrolyzed by adding thereto an acid in a prescribed amount. Accordingly, the use of a large amount of an alkaline metal hydroxide and the neutralization step for obtaining N-phosphonomethylglycine after hydrolysis, which steps are necessary in a conventional alkali hydrolysis method, are omitted in the present invention.