PURPOSE: To efficiently obtain the subject compound useful as an acylating agent for acylating amines, etc., under mild conditions without using mercury, by reacting a carboxylic acid with an alkoxyacetylene in the presence of a ruthenium catalyst.

CONSTITUTION: A carboxylic acid(e.g. acetic acid) of formula: R1COOH (R1 is alkyl, aryl, aralkyl) is reacted with an alkoxyacetylene (preferably ethoxyacetylene) of formula I (R2 is alkyl) in the presence of a ruthenium catalyst [preferably dichloro (η-p-pcymene) ruthenium (II)] preferably at 0-40°C to obtain the objective compound of formula II. The ruthenium catalyst is preferably used in a molar ratio of 0.003-0.05X based on the carboxylic acid.