A power supply circuit design system according to an exemplary aspect of the invention includes: a power supply voltage fluctuation deriving means for deriving a power supply voltage fluctuation characteristic as a voltage fluctuation characteristic in a semiconductor integrated circuit on the basis of design information about a power supply circuit for connecting the semiconductor integrated circuit and other components mounted on a substrate; a determination reference database including a power supply voltage fluctuation condition as a condition for which the power supply voltage fluctuation characteristic is allowed in the power supply circuit, and a change indicator for at least one of a circuit structure and operation of the semiconductor integrated circuit; a power supply voltage fluctuation determination means for comparing the power supply voltage fluctuation characteristic and the power supply voltage fluctuation condition, and determining whether the power supply voltage fluctuation characteristic satisfies the power supply voltage fluctuation condition; and a circuit structure changing means for changing at least one of the structure and the operation of the semiconductor integrated circuit in accordance with the change indicator if the power supply voltage fluctuation characteristic does not satisfy the power supply voltage fluctuation condition, and outputting design information about the changed semiconductor integrated circuit to the power supply voltage fluctuation deriving means; wherein the change indicator does not entail a change in size of the semiconductor integrated circuit.