A compound represented by the following formula (1) or a salt thereof: where R 1 represents a hydrogen atom, an alkyl group with a number of carbon atoms from 1 to 6, an alkyloxy group with a number of carbon atoms of 1 to 6, or a halogen atom； R2 represents a hydroxyl group, or an alkyloxy group with a number of carbon atoms of 1 to 6； R3 represents a hydrogen atom, an alkyl group with a number of carbon atoms from 1 to 6, or a halogen atom； R4 represents a hydrogen atom or an alkyloxy group with a number of carbon atoms from 1 to 6； R5 represents a hydrogen atom, an alkyloxy group with a number of carbon atoms from 1 to 6, a halogen atom, a hydroxyl group, or a methylenedioxy group formed together with R6, or an isopropylidenedioxy group formed together with R6； R6 represents a hydrogen atom, an alkyloxy group with a number of carbon atoms from 1 to 6, or a methylenedioxy group formed together with R5 or an isopropylidenedioxy group formed together with R5； R7 represents a hydrogen atom or an alkyl group with a number of carbon atoms from 1 to 6； and R8 represents a hydrogen atom, a hydroxyl group, an amino group, or an alkylcarbonyloxy group with a number of carbon atoms of 1 to 6； with the proviso that the following cases are excluded: the case where R1, R3, R4 and R7 are hydrogen atoms； R2 and R8 are hydroxyl groups and R5 and R6 are methoxy groups； the case where R1, R3, R4 and R7 are hydrogen atoms, R2, R5 and R6 are methoxy groups, and R8 is a hydroxyl group； the case where R1, R3, R4, R7 and R8 are hydrogen atoms and R2, R5 and R6 are methoxy groups； the case where R1, R3, R4 and R7 are hydrogen atoms, R2, R5 and R8 are hydroxyl groups, and R6 is a methoxy group； the case where R1, R3, R4 and R7 are hydrogen atoms, R2 and R6 are methoxy groups, and R5 and R8 are hydroxyl groups； the case where R1, R3, R4, R7 and R8 are hydrogen atoms, R2 and R6 are methoxy groups, and R5 is a hydroxyl group； the case where R1, R2, R5 and R6 are methoxy groups and R3, R4, R7
