Provided is a noise equivalent circuit for completing an EMC analysis in a practical time and through a low-cost calculation process at an upstream stage of system design. The noise equivalent circuit includes one or more energy sources; a propagation path for propagation of energy from the energy source including a conductive path such as a cable and an electromagnetic field coupling path due to the coupling of an electric field and a magnetic field with another electronic device or cable; and a GND port connected to a system, and each port is represented by the noise voltage source or the noise current source and the internal impedance. This noise equivalent circuit can determine an external impedance that is varied depending on a load connected externally or the distance from an external device or a cable, whereby the noise of the system as a whole can be analyzed.