A 3-anilino-2-cycloalkenone derivative of the formula (I): wherein, R1 represents a C1 to C8 alkyl group, which may have a substituent, except for a methyl group, a C3 to C7 cycloalkyl group, a 3-tetrahydrofuryl group, an 2-indanyl group, etc., R2 represents a C1 to C4 alkyl group, R3 represents a hydrogen atom, a C3 to C7 alkyl group, which may have a substituent, a C3 to C7 cycloalkyl group, etc., R4 represents a hydrogen atom, a C1 to C5 alkyl group, which may have a substituent, a halogen atom, etc., R5, R6, R7, and R8 independently represent a hydrogen atom, a C1 to C5 alkyl group, which may have a substituent, etc., X represents —(CR11R12)n—, wherein n is 0 to 2, R11 and R12 independently represent a hydrogen atom, a C1 to C5 alkyl group, which may have a substituent, etc. or —NR13— wherein R13 represents a hydrogen atom or a C1 to C5 alkyl group, which may have a substituent, and its optical isomers or their pharmaceutically acceptable salts or their hydrates or solvates.