To provide a semiconductor device which can be stably operated while achieving a reduction of the power consumption.A semiconductor device includes a CPU, a system controller which designates an operation speed of the CPU, P-type SOTB transistors, and N-type SOTB transistors. The semiconductor device is provided with an SRAM which is connected to the CPU, and a substrate bias circuit which is connected to the system controller and is capable of supplying substrate bias voltages to the P-type SOTB transistors and the N-type SOTB transistors. Here, when the system controller designates a low speed mode to operate the CPU at a low speed, the substrate bias circuit supplies the substrate bias voltages to the P-type SOTB transistors and the N-type SOTB transistors.