Abstract: The invention provides a method to search for a best promoter for reliably targeting a cancerization-causing cell in human ES/iPS cells. Discovering a gene which can kill or remove a cancerization-causing cell contained in human ES/iPS cells with high efficiency is also provided. A method is provided for removing an undifferentiated cell, which remains after the differentiation of human ES/iPS cells into desired cells, with high efficiency and comprehensively. A viral vector is provided which includes a nucleotide sequence and a recombination cassette. The nucleotide sequence contains a target gene and a killing gene that are linked to each other through a sequence that enables the simultaneous expression of the two genes by one promoter. The recombination cassette contains a promoter region which is so linked as to enable the expression of the labeling gene and the killing gene. The viral vector may contain a promoter specific to an undifferentiated cell.