An object of the present invention is to expand CD34 + cells ex vivo efficiently in a short term using a biologically safe and inexpensively obtainable low molecular weight compound. A still another object of the present invention is to provide an expansion agent for CD34 + cells useful for treatment of various hematopoietic disorders caused by dysfunctional hematopoietic stem cells and/or hematopoietic progenitor cells. A method for expanding CD34 + cells, which comprises culturing CD34 + cells ex vivo in the presence of a compound represented by the formula (I) (wherein A, B, L 1 , L 2 , L 3 , L 4 , R 1 , R 2 , R 3 , X and Y are defined in the description), a tautomer or pharmaceutically acceptable salt of the compound, or a solvate thereof.