The present invention provides a compound of formula (1) or a salt thereof wherein ring Q1 is optionally-substituted C6-10 aryl, etc.; R1 and R2 are independently hydrogen atom, etc.; W1 is C1-4 alkylene which may be optionally substituted with 1 to 3 fluorine atoms or C3-7 cycloalkyl; W2 is —NR4aC(O)—, etc. wherein R4a is hydrogen atom or C1-6 alkyl; ring Q2 is optionally-substituted C6-10 aryl, etc., which has an inhibitory effect on the sphere-forming ability of cancer cells and is useful an agent for removing iPS cells.