PURPOSE: To obtain a compound useful as a methylesterifying agent keeping effect for a long period by simple operation in high yield, by purifying diazomethane with an ether and dissolving in another ether.

CONSTITUTION: p-Toluenesulfonyl-N-methyl-N-nitrosoamide is reacted with an alcohol such as methanol, etc., in the presence of an alkali (e.g. NaOH) to give a diazomethane gas. Then the gas is brought into contact with dimethyl ether and, after the contact, the diazomethane gas is passed through a trap to remove impurities. Further the gas is dissolved in another ether to give the aimed substance. The concentration of diazomethane in the ether solution is 1W5wt.% and the solution preferably has appearance of showing light lemon.