The present invention relates to a compound represented by Formula [I] or a pharmaceutically acceptable salt or ester thereof: wherein: X1, X2, X3, and X4, which may be identical or different, are each C or N, provided that none to two of X1, X2, X3, and X4 is/are N; Y is CH or N; R1, R1′, R2, R2′, R3, R3′, R4, and R4′, which may be identical or different, are each a hydrogen atom, a lower alkyl group, or the like; R5 is a hydrogen atom or a methyl group; R6 and R7, which may be identical or different, are each a hydrogen atom, a lower alkyl group, or the like; R8 and R8′, which may be identical or different, are each a hydrogen atom, a lower alkyl group, or the like; R9 is an aryl group or a heteroaryl group which may be substituted; and n is an integer from 1 to 3, and a PLK1 inhibitor or an anticancer agent containing the same.