The carbostyril derivative or pharmaceutically acceptable salt thereof according to claim 1, wherein R1 is a lower alkyl group, a lower alkenyl group, a lower alkynyl group or a phenyl-lower alkyl group; and R2 is a lower alkanoyl group, a lower alkoxycarbonyl group, a furoyl group, a lower alkanesulfonyl group, a benzoyl group (which benzoyl group may have (a) 1 to 3 substituents on the phenyl ri