A benzoylaminophenoxybutanoic acid derivative of general formula: ##STR1## [wherein A represents oxygen atom, sulfur atom or sulfinyl (SO) group,R.sup.1 represents hydrogen atom or methyl group,R.sup.2 represents a group of the general formula: ##STR2## {wherein B represents oxygen atom, sulfur atom or a group of general formula: NR" (wherein R" represents hydrogen atom or alkyl group of from 1 to 4 carbon atoms(s)), R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7 and R.sup.8 represent, independently, hydrogen atom, alkyl group of from 1 to 4 carbon atom(s), halogen atom or trifluoromethyl group,m represents 0 or 1,n represents an integer of from 1 to 4, andR.sup.9 and R.sup.10 represent, independently, hydrogen atom, alkyl group of from 1 to 5 carbon atom(s) or a group of general formula: ##STR3## (wherein R.sup.12, R.sup.13, R.sup.14 and R.sup.15 represent, independently, hydrogen atom, alkyl group of from 1 to 4 carbon atom(s), halogen atom or trifluoromethyl group, and 1 represents an integer of from 1 to 4.).With the proviso that R.sup.9 and R.sup.10 do not represent hydrogen atoms at the same time.}.] or non-toxic salts thereof possess an inhibitory activity on 5.alpha.-reductase, and therefore be useful for treating and/or preventing agent for alopecia, acnes or prostatic hypertrophy.