The present invention provides a carbostyril compound represented by General Formula (1) or a salt thereof, wherein A is a direct bond, a lower alkylene group, or a lower alkylidene group; X is an oxygen atom or a sulfur atom; R4 and R5 each represent a hydrogen atom; the bond between the 3 and 4 positions of the carbostyril skeleton is a single bond or a double bond; R1 is a hydrogen atom, etc; R2 is a hydrogen atom, etc; and R3 is a hydrogen atom, etc. The carbostyril compound or salt thereof of the present invention induces the production of TFF, and thus is usable for the treatment and/or prevention of disorders such as alimentary tract diseases, oral diseases, upper respiratory tract diseases, respiratory tract diseases, eye diseases, cancers, and wounds.