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 NY is CH or NR1, 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 likeR5 is a hydrogen atom or a methyl groupR6 and R7, which may be identical or different, are each a hydrogen atom, a lower alkyl group, or the likeR8 and R8, which may be identical or different, are each a hydrogen atom, a lower alkyl group, or the likeR9 is an aryl group or a heteroaryl group which may be substituted andn is an integer from 1 to 3,and a PLK1 inhibitor or an anticancer agent containing the same.
