1. A parameter estimation device comprising: an input-signal detecting means for detecting an input signal inputted to an object system that is capable of being expressed by an equivalent circuit model containing at least an integrator element; an output-signal detecting means for detecting an output signal outputted from the object system; an output differentiating means that differentiates the output signal detected by the output-signal detecting means to output an output differential signal; a modified equivalent circuit model of the object system in which each element of the equivalent circuit model is changed into non-integrator element in such a way that the output signal obtained when the input signal outputted from the input-signal detecting means becomes to be the output differential signal; and an element parameter estimating means for estimating parameters corresponding to respective elements of the modified equivalent circuit model based on the modified equivalent circuit model by receiving and using the output differential signal and the input signal that are inputted to the modified equivalent circuit model.