An object of the present invention is to provide a battery state estimation device and a battery state estimation method which identify parameter of a secondary battery with high accuracy. A current and a terminal voltage of secondary battery are detected, then by using the current measured value and terminal voltage measured value thus detected, the terminal voltage of secondary battery based on predetermined battery model is estimated, and the parameter of the secondary battery is identified such that a difference between terminal voltage measured value and voltage estimated value is converged to zero. In the identifying of parameter of secondary battery, the terminal voltage measured value and terminal voltage estimated value are subjected to filter treatment by low pass filter having a common high frequency breaking characteristic and the terminal voltage measured value and terminal voltage estimated value subjected to the filter treatment are used.