A gene coding for a mutant tyrosine repressor having increased positive regulatory activity for expression of the tyrosine phenol lyase gene compared with a tyrosine repressor not having mutation is obtained by introducing a mutation into a tyrosine repressor gene of Erwinia herbicola, introducing the gene into which the mutation is introduced into Escherichia coli expressing a lactose operon under the control of a promoter and enhancer of the tyrosine phenol lyase gene derived from Erwinia herbicola, and selecting a transformed strain having increased -galactosidase activity.