Transformed cells are efficiently selected using a mutant ALS gene having high specificity to PC herbicides. The transformation method comprises the steps of: transforming a host cell with a recombination vector containing a gene of interest and a gene coding for a mutant acetolactate synthase having mutation of glycine corresponds to position 95 of the amino acid sequence of a wild-type acetolactate synthase derived from rice to alanine; culturing the transformed cell obtained in the former step in the presence of a pyrimidinyl carboxy herbicide; and wherein the gene coding for the mutant acetolactate synthase is used as a selection marker.