A compound represented by the following formula (I) or a salt thereof: wherein R1, R2, R3 and R4 represent hydrogen atom, a halogen atom, cyano group, nitro group, a C1-4 alkyl group, a halogenated C1-4 alkyl group or a C1-4 alkoxy group, R5 represents a halogen atom, cyano group, a C1-4 alkyl group, a halogenated C1-4 alkyl group or a C1-4 alkoxy group, R6 represents a C5-7 cycloalkyl group, a substituted C5-7 cycloalkyl group, a 5 to 7-membered completely saturated heterocyclic group or a substituted 5 to 7-membered completely saturated heterocyclic group, X represents a single bond, oxygen atom, sulfur atom, NR7, —O—CH2— or —N(R8)—CH2—, R7 represents hydrogen atom or a C1-4 alkyl group, or R7 may combine with a substituent of R6 to represent a single bond, methylene group or ethylene group, R8 represents hydrogen atom, a C1-4 alkyl group or a C7-12 aralkyl group, which is useful as an active ingredient of a medicament for prophylactic and/or therapeutic treatment of diseases caused by an activation of STAT6 and/or NF-κB.