A phosphonic acid derivative compound represented by formula [I ] or a pharmaceutically acceptable salt thereof: ##STR1## wherein R.sub.1, R.sub.2 and R.sub.3 each represent hydrocarbon groups which may be substituted, except cases in which (1) R.sub.2 is unsubstituted methyl, ( 2 ) R.sub.3 is an unsubstituted hydrocarbon group having 1 to 3 carbon atoms, and (3) R.sub.1 is benzyloxycarbonylaminomethyl, R.sub.2 is isobutyl and R.sub.3 is isobutyl or phenylmethyl, which has endothelin-converting enzyme inhibiting activity and is useful as pharmaceutical drugs such as therapeutic agents for hypertension, cardiac or cerebral circulatory diseases and renal diseases.