Abstract: An expansion agent for hematopoietic stem cells and/or hematopoietic progenitor cells useful for improvement in the efficiency of gene transfer into hematopoietic stem cells for gene therapy useful for treatment of various hematopoietic disorders is provided. A method for expanding hematopoietic stem cells and/or hematopoietic progenitor cells, which comprises culturing hematopoietic stem cells and/or hematopoietic progenitor cells ex vivo in the presence of a compound represented by the formula (I) (wherein X, Y, X, Ar1, R1, R2, R3, R4, R5, R6 and R7 are defined in the description), a tautomer or pharmaceutically acceptable salt of the compound or a solvate thereof.