The intermediate products yielded by the reaction of biguanides with diethyl oxalate were inferred to be (4, 5-dioxo-2-imidazolidinylidene) guanidines by means of their infrared absorption spectra. Moreover, in order to elucidate this reaction mechanism, the reaction between guanidine containing the similar structural moiety to biguanides and diethyl oxalate was examined. As the result, it was assumed that the reaction of biguanide with diethyl oxalate would proceed through the cyclic transition state to afford (4, 5-dioxo-2-imidazolidinylidene) guanidine.