The present inventors found that NEK2 kinase (accession number NM_002497) is expressed specifically in tumor cells such as bile duct carcinoma cells and that repression of the expression of NEK2 kinase by the use of RNA interference method led to inhibition of the growth of the tumor cells. A cytostatic agent for bile duct carcinoma and others containing an oligoribonucleotide comprising a segment of bases identical or complementary to less than 30 consecutive bases of nucleotide sequence of SEQ ID NO: 1, which includes at least 19 consecutive nucleotides of bases 403-423, 507-527 or 989-1009 of SEQ ID NO: 1 (NEK2 gene), or double-stranded RNA composed of the oligoribonucleotides.
