An agent for preventing/treating respiratory diseases contains, as an active ingredient, a compound represented by following Formula (I): ##STR00001## wherein A is a group represented by L-W [wherein L is a bond or CH.sub.2; and W is O, SOn (wherein n is 0 to 2), or --NR.sup.7-- (wherein R.sup.7 is hydrogen or lower alkyl)]; each of G.sup.1 and G.sup.2 is (CH.sub.2)r (wherein r is 0 to 2), provided that when n is 1, G.sup.1 and G.sup.2 may be bridged by lower alkylene; Y is a lower alkylene or (substituted) benzylidene; Z is a bond or O, provided that when Z is a bond, Y may form a 5- or 6-membered ring with carbon on the benzene ring; R.sup.1 is, for example, NO.sub.2, a lower alkoxycarbonyl, (substituted) carbamoyl, (protected) hydroxyl group, (protected) carboxyl, or (protected) N-hydroxycarbamoyl; each of R.sup.2 and R.sup.3 is hydrogen, halogen, (halogenated) lower alkyl, (halogenated) lower alkoxy or NO.sub.2; each of R.sup.4 and R.sup.5 is, for example, hydrogen, halogen, (halogenated) lower alkyl, (halogenated) lower alkoxy, CN, or lower alkylsulfonyl; and R.sup.6 is hydrogen or lower alkyl, a salt thereof or a solvate of them. It has excellent antitussive activity when used as an agent for preventing/treating respiratory diseases such as lung cancer, common cold syndrome, pulmonary tuberculosis, pneumonia, acute bronchitis or chronic bronchitis.