PURPOSE: To obtain sulfonylurea in a high yield which is useful as a herbicide, by a new method by reacting sulfonamide with an alkali (earth) metal oxide and thereafter reacting the obtained product with a specified urethane (cardamate).

CONSTITUTION: At first, a sulfonamide of the formula R1-SO2-NH2 (R1 is an aryl, aralkyl or the like) is reacted with an alkali (earth) metal hydroxide or an alkali (earth) metal alcoholate optionally in the presence of a diluent at a temp. of -20 to 150°C, to obtain a sulfonamide salt of the formula R1-SO2-NH- M+ [M+ is an alkali (earth) metal equivalent]. Then, the salt is reacted with urethane (carbamate) of formula I (R2 is H, an alkenyl or the like; R3 is an alkyl, aralkyl or the like; X is H, an alkylthio or the like; Y is H, a halogen or the like; Z is N, CH or the like) optionally in the presence of a diluent a temp. of at -20 to 150°C. Thereafter the objective compd. of formula II is obtained from the reaction mixture according to any conventional method.