PURPOSE: To increase capacitance of a power supply line without increasing an area of a semiconductor device so as to suppress a fluctuation in potential by forming a power source capacitor using an unused function cell in a logic circuit.

CONSTITUTION: An unused electrode 2 among electrodes placed at an end of a semiconductor substrate 1 is used to construct a capacitor 20. That is, the unused electrode 2 comprises the first electrode plate 21 formed on the substrate 1 with an insulation layer 5 interposed similarly to a wiring layer of the first layer and the second electrode plate 22 formed on the first electrode plate 21 with the insulation layer 5 further interposed. Thus a power supply capacitor is formed without giving influence to an area and a structure of a semiconductor device to increase capacitance of a power source circuit, so that a fluctuation in potential can be suppressed even if there is a sudden fluctuation in consumption current.