A compound or a pharmaceutically acceptable salt thereof of the present invention is represented by the following general formula (I): [wherein, R1 to R8 may have a hydrogen atom, a halogen atom, a hydroxy group, a C1-C6 alkyl group, a C2-C8 alkenyl group, a C1-C6 alkylcarbonyl group or —COOR9 (wherein R9 represents a hydrogen atom, a C1-C6 alkyl group or a C2-C8 alkenyl group) as a substituent; and X represents a sulfur atom, an oxygen atom or NR10 (wherein R10 represents a hydrogen atom, a halogen atom, a hydroxy group, a C1-C6 alkyl group, a C2-C8 alkenyl group or a C1-C6 alkoxy group)].