A semiconductor device includes a first power supply line; a second power supply line; a first cell arrangement area in which a first cell is arranged; and a switch area in which a switching transistor and a decoupling capacitance are arranged. The first cell is provided in a first well of a first conductive type, the switching transistor is provided in a second well of the first conductive type, and the decoupling capacitance is provided in a separation area of a second conductive type to separate the first well and the second well from each other. The switching transistor connects the first power supply line and the second power supply line in response to a control signal, the first cell operates with power supplied from the second power supply line, and the decoupling capacitance is connected with the first power supply line.