The present invention provides: a 1,4-di-substituted imidazole derivative that is represented by formula (1'), that shows excellent cancer cell sphere forming ability suppressing effect, and that is useful as an orally administrable antitumor agent； and a pharmaceutically acceptable salt thereof. [In the formula, ring Q1 represents an optionally substituted C6-10 aryl group or the like, R1 and R2 independently represent a hydrogen atom or the like, W1 represents an optionally substituted C1-4 alkylene group, W2 represents -NR3aC(O)- or the like (wherein, R3a represents a hydrogen atom or a C1-6 alkyl group)； ring Q2 represents a 5-10 membered heteroaryl group； W3 represents an optionally substituted C1-4 alkylene group or the like； n represents 1, 2, 3, 4, or 5； R4 represents, independently when more than one thereof exists, a hydrogen atom, an optionally substituted C1-6 alkyl group, or the like； and R5 represents a hydroxy group or the like.]
