The present invention relates to a compound represented by Formula [I] or a pharmaceutically acceptable salt or ester thereof: ##STR00001## wherein: X.sub.1, X.sub.2, X.sub.3, and X.sub.4, which may be identical or different, are each C or N, provided that none to two of X.sub.1, X.sub.2, X.sub.3, and X.sub.4 is/are N; Y is CH or N; R.sub.1, R.sub.1', R.sub.2, R.sub.2', R.sub.3, R.sub.3', R.sub.4, and R.sub.4', which may be identical or different, are each a hydrogen atom, a lower alkyl group, or the like; R.sub.5 is a hydrogen atom or a methyl group; R.sub.6 and R.sub.7, which may be identical or different, are each a hydrogen atom, a lower alkyl group, or the like; R.sub.8 and R.sub.8', which may be identical or different, are each a hydrogen atom, a lower alkyl group, or the like; R.sub.9 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.