The present invention provides an expression vector, containing expressibly (a) a polynucleotide encoding a fusion protein of a mutant of a repressor protein, which binds to an antibiotic, and a target protein, and (b) a polynucleotide encoding a protein controlling the transcription of the polynucleotide in (a), the transcription of the polynucleotide in (a) and the degradation of said fusion protein, which is the expression product of the polynucleotide in (a), being controlled inside a cell by the presence or absence of an antibiotic inside the cell.
