A process for preparing an enol silyl ether compound from a diazoacetoacetic acid ester having the general formula (IV): ##STR1## wherein R.sup.1 is a lower alkyl group having 1 to 6 carbon atoms, phenyl group, a substituted phenyl group, an aralkyl group or allyl group, and R.sup.2, R.sup.3 and R.sup.4 are the same or mutually different and each is a lower alkyl group having 1 to 6 carbon atoms, which comprises reacting a diazoacetoacetic acid ester having the general formula (I): ##STR2## wherein R.sup.1 is the same as defined above, with a trialkylsilyl chloride having the general formula (II): ##STR3## wherein R.sup.2, R.sup.3 and R.sup.4 are the same as defined above, in an inert solvent in the presence of an organic base and an alkali halide having the general formula (III):MX (III)wherein M is an alkaline metal and X is bromine atom or iodine atom. The desired compound is useful as an intermediate for synthesis of carbapenem .beta.-lactam antibiotics.