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 disorders is provided. An expansion agent for hematopoietic stem cells and/or hematopoietic progenitor cells is provided, containing a compound represented by the formula (I): (wherein X, Y, Z, Ar1 , R1, R2, R3, R4, R5, R6 and R7 are as defined in the description), a tautomer, amide, ester, or pharmaceutically acceptable salt of the compound or a solvate thereof, which can expand hematopoietic stem cells and/or hematopoietic progenitor cells.