An equivalent circuit is capable of, while having a simple configuration, accurately expressing a superposition characteristic and having excellent practicality and workability. A current sensor and a voltage source are connected in series between external terminals of an equivalent circuit. A reference state element having an impedance forms a closed loop with a current source. A current flowing through the equivalent circuit and detected by the current sensor is reproduced by the current source and then applied to the reference state element, so that a potential difference is generated across the reference state element. A voltage obtained by multiplying the potential difference by a correction coefficient is outputted by the voltage source. By setting the correction coefficient to be dependent on the current or an inter-terminal voltage, the impedance represented by the equivalent circuit can be reproduced as a characteristic dependent on the current I or the inter-terminal voltage.