1. A method of deriving equivalent circuit model, comprising the steps of: providing impedance Z(ƒ n ) at each of sample frequencies (f 1,..., f N, where f 1 <f N ), the impedance being given as Z (ƒ n )&equals; R (ƒ n )&plus; jX (ƒ n ), where Z is the impedance of capacitor, R is a real part of Z, X is an imaginary part of Z, ƒ n is the sample frequencies (n&equals;1, 2,..., N), and j is the imaginary unit; forming an equivalent circuit model by adopting one of an RC circuit consisting of a resistance and a capacitance, an RL circuit consisting of a resistance and an inductance, and an RCL circuit consisting of the RC circuit and the RL circuit connected in series; composing an evaluation function Q({overscore (P)}) in accordance with formulae: 16 Q &af; ( P _ ) = &Sum; n = 1 m &it; q &af; ( R M &af; ( f n, P _ ), X M &af; ( f n, P _ ), R &af; ( f n ) - R 0 / 2, X &af; ( f n ) ), and q &af; ( R M, X M, R, X ) = c R &it; | R M - ( R - R 0 / 2 ) &it; | 2 | R &it; | d + c X &it; | X M - X &it; | 2 | X &it; | d + c Z &it; | Z M - Z &it; | 2 | Z &it; | d where an impedance of the equivalent circuit model is defined as Z M (ƒ n, {right arrow over (P)})&equals; R M (ƒ n, {right arrow over (P)})&plus; jX M (ƒ n, {right arrow over (P)}), in which Z M is the impedance of the equivalent circuit model, X M is and imaginary part of Z M, f n is each sample frequency (n&equals;1, 2,..., N). j is the imaginary unit, {right arrow over (P)}&equals;(P 1, P 2,... P K ) is a circuit constant vector in the circuit having components in element values of R, C and/or L, and C R, C X, and C Z are positive real numbers or zero; and determining the circuit constant vector {right arrow over (P)} by minimizing the evaluation function Q({right arrow over (P)}).