Tumor growth was found to be significantly suppressed in vivo by inhibiting both miRNA containing 5'-AACACUG-3' as a seed sequence and miRNA containing 5'-AAUACUG-3' as a seed sequence. The inhibition significantly altered the proportion of subpopulations of tumor cells and reduced the tumorigenicity in all subpopulations. The inhibition also exerted a remarkable tumor-shrinking effect on already-formed tumors. The present invention provides novel therapeutic potential against tumor.