PURPOSE: To provide the state estimation observer device which can estimate the state of even a control system, which includes parasitic elements such as a high-order mode and an idle time, stably at all times.

CONSTITUTION: This device A consists of a dynamic characteristic determination part 2 which determines both dynamic characteristics of the 1st route and the 2nd route so that the product of the 1st parasitic elements PI(S), PO(S) which are present in the 1st route from a control input (u) to an output observed value yS, and the dynamic characteristics NY(S) of the 1st route becomes equal to the product of the 2nd parasitic element PY(S) present in the 2nd route from the control input (u) to an input observed value uS and the dynamic characteristics NU(S) of the 2nd route, an state estimation part 3 which estimates the state of a controlled system 1 by using the value yD obtained by multiplying the outputted observed value yS by the dynamic characteristics NY of the 1st route determined by the dynamic characteristic determination part 2 and the value uD obtained by multiplying the input observed value uS by the dynamic characteristics NU(S) of the 2nd route determined by the dynamic characteristic determination part 2, etc. Consequently, the state of even the control system including the parasitic elements such as the high-order mode and idle time can be estimated stably at all times.