In a lactose repressor protein, at least one amino acid at the position of 94, 241, 265 or 300 in the wild lactose repressor is replaced with an amino acid other than that of the wild lactose repressor, and the use thereof. The present mutant lactose repressor represses the expression of a desired gene at 30°C or lower temperature, and induces the expression of a desired gene at 37°C or higher temperature, and therefor can control the expression by change of a culture temperature without using an expensive inducer such as IPTG.