There is provided an indole derivative represented by the above formula (I) or a pharmaceutically acceptable salt thereof which is useful for the treatment of malignant tumor or a brain neurodegenerative disease: (wherein C ring represents a benzene ring or a cyclohexene ring; X and Y are the same or different and each represent —CH2—, —CH(OH)—, —CH(ORx)—, —CH(SRY)— or carbonyl; R1 and R2 are the same or different and each represent a hydrogen atom, substituted or unsubstituted lower alkyl, substituted or unsubstituted lower alkenyl or substituted or unsubstituted lower alkanoyl or R1 and R2 form a benzene ring together two carbon atoms each being adjacent thereto; and R3, R4 and R5 are the same or different and each represent a hydrogen atom, substituted or unsubstituted lower alkyl, etc.)