The present invention provides a compound of the following Formula (1) or a pharmaceutically acceptable salt thereof: wherein X is an oxygen atom and the like; Y is —CO—, —SO2— and the like; R1 is an optionally-substituted C1-6 alkyl group, an optionally-substituted C1-6 alkylcarbonyl group and the like; R2 is an optionally-substituted C1-6 alkyl group, an optionally-substituted C1-6 alkoxy group, an optionally-substituted amino group, an optionally-substituted 5- to 12-membered monocyclic or polycyclic saturated heterocyclic group and the like; R3, R4, R5, and R6 are independently a hydrogen atom and the like which exhibits excellent effects in suppressing the proliferation and sphere-forming ability of cancer cells, and can be useful as an antitumor drug or cell growth inhibitor.