A substrate bias is controlled such that a leakage current is minimum. A semiconductor integrated circuit device comprises a leakage detecting circuit which detects a leakage current by using leakage detecting MOSFETs, a control circuit which generates a control signal depending on an output from the leakage detecting circuit, a substrate bias generating circuit which changes a substrate bias depending on the control signal, and a controlled circuit including a MOSFET having the same characteristics as that of each of the leakage detecting MOSFETs. The leakage detecting circuit detects a substrate leakage current which includes as the substrate bias becomes deep and a subthreshold leakage current which decreases as the substrate bias becomes deep. A control signal is transmitted to the substrate bias generating circuit such that the substrate bias is made deep when the substrate leakage current is smaller than the subthreshold leakage current and such that the substrate bias is made shallow when the substrate leakage current is larger than the subthreshold leakage current.