The invention provides a method of removing or reducing undifferentiated cells from a differentiated cell population contaminated or having a risk of contamination with undifferentiated cells by contacting a pigment epithelium-derived factor with the differentiated cell population to induce apoptosis of the undifferentiated cells. The invention also provides an agent for cell transplantation therapy, containing a differentiated cell population substantially free of an undifferentiated cell, which is obtained by the method, as well as an agent for inducing apoptosis of an undifferentiated cell, containing a pigment epithelium-derived factor, and combined use of the aforementioned agent for cell transplantation therapy and the aforementioned agent for inducing apoptosis.
