The present invention provides a lethal agent for undifferentiated cells and/or cells that would cause tumorigenesis, the lethal agent containing a virus vector in which a promoter for a nucleic acid for encoding an essential factor for replicating or assembling at least one virus has been replaced with a promoter for a gene which is specifically expressed in cancer cells or undifferentiated cells. The present invention further provides a lethal agent for undifferentiated cells and/or cells that would cause tumorigenesis, the lethal agent containing an expression vector including a nucleic acid for encoding a cytotoxic factor under the control of a promoter for a gene which is specifically expressed in cancer cells or undifferentiated cells. The present invention additionally provides a method for producing differentiated cells having a reduced risk of tumorigenesis, the method being characterized in that an aforesaid drug is brought into contact with a cell population induced to differentiate from stem cells
