A compound of the general formula: ##STR1## wherein R.sup.1 represents a carboxyl group which may optionally be esterified or amidated; R.sup.2 represents a cyclic group which may optionally be substituted or a polar group; n is an integer of 0 to 6; R.sup.3 represents hydrogen or a hydrocarbon residue which may optionally be substituted; R.sup.4 represents (1) a hydrocarbon residue which is substituted by an optionally protected amino group or (2) an alkenyl group; or R.sup.3 and R.sup.4 may be combined with the adjacent nitrogen atom to form a heterocyclic group containing at least two hetero atoms, or a salt thereof. The compound or a salt thereof of the present invention inhibits thiol proteases such as cathepsin L and B and serves well as a prophylactic/therapeutic agent for bone diseases such as osteoporosis.