An agent for preventing / treating respiratory diseases contains, as an active ingredient, a compound represented by following Formula (I):wherein A is a group represented by L-W [wherein L is a bond or CH2; and W is O, SOn (wherein n is 0 to 2), or —NR7— (wherein R7 is hydrogen or lower alkyl)]; each of G1 and G2 is (CH2)r (wherein r is 0 to 2), provided that when n is 1, G1 and G2 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; R1 is, for example, NO2, a lower alkoxycarbonyl, (substituted) carbamoyl, (protected) hydroxyl group, (protected) carboxyl, or (protected) N-hydroxycarbamoyl; each of R2 and R3 is hydrogen, halogen, (halogenated) lower alkyl, (halogenated) lower alkoxy or NO2; each of R4 and R5 is, for example, hydrogen, halogen, (halogenated) lower alkyl, (halogenated) lower alkoxy, CN, or lower alkylsulfonyl; and R6 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.