A semiconductor device includes a circuit comprising a first transistor in a first Fin; a power supply circuit in a second Fin, the power supply circuit comprising a second transistor connected between the circuit and a power supply line; and a substrate contact electrically connected to the semiconductor substrate and configured to apply a substrate voltage to a substrate, wherein a width of the first Fin in a cross-section of the first Fin perpendicular to a channel length direction of the first transistor is equal to or smaller than a twofold of a largest depletion layer width of a depletion layer formed in a channel part of the first transistor, and a width of the second Fin in a cross-section of the second Fin perpendicular to a channel length direction of the second transistor is larger than a twofold of a largest depletion layer width of a depletion layer in a channel of the second transistor.