A parameter estimation device using a filter comprising: a discharge-and-charge current detecting means for detecting a discharge-and-charge current value of a battery; a terminal voltage detecting means for detecting a terminal voltage value of the battery; a state-of-health detecting means for detecting a state of health of the battery; a preprocessing computing means configured to transfer the discharge-and-charge current value detected by the discharge-and-charge current detecting means and the terminal voltage value detected by the terminal voltage detecting means to a current preprocessing value and a voltage preprocessing value, respectively, by inputting the discharge-and-charge current value and the terminal voltage value to a low-pass filter; a state estimating means configured to estimate parameters of a state equation expressing a battery equivalent circuit model from the voltage preprocessing value and the current preprocessing value obtained by the preprocessing computing means, based on the battery equivalent circuit model containing a resistance and a capacitor; and a state-of-charge estimating means configured to estimate a state of charge of the battery based on a state quantity obtained by the state estimating means, wherein the low-pass filter has a time constant that is set to vary in such a way that the higher the state of health detected by the state-of-health detecting means becomes the smaller the time constant becomes, and such that the higher the state of charge estimated by the state-of-charge estimating means becomes the smaller the time constant becomes, characterized by the state-of-health detecting means being adapted to detect a state of health of the battery by inputting the discharge-and-charge current value detected by the discharge-and-charge current detecting means and the terminal voltage value detected by the terminal voltage detecting means, wherein a smaller detected state-of-health represents a more severe deterioration of the battery; the device further comprising a temperature detecting means for detecting a temperature of the battery; and the time constant of the low-pass filter is further set to vary in such a way that the higher the temperature detected by the battery temperature detecting means the smaller the time constant becomes. a discharge-and-charge current detecting means for detecting a discharge-and-charge current value of a battery; a terminal voltage detecting means for detecting a terminal voltage value of the battery; a state-of-health detecting means for detecting a state of health of the battery; a preprocessing computing means configured to transfer the discharge-and-charge current value detected by the discharge-and-charge current detecting means and the terminal voltage value detected by the terminal voltage detecting means to a current preprocessing value and a voltage preprocessing value, respectively, by inputting the discharge-and-charge current value and the terminal voltage value to a low-pass filter; a state estimating means configured to estimate parameters of a state equation expressing a battery equivalent circuit model from the voltage preprocessing value and the current preprocessing value obtained by the preprocessing computing means, based on the battery equivalent circuit model containing a resistance and a capacitor; and a state-of-charge estimating means configured to estimate a state of charge of the battery based on a state quantity obtained by the state estimating means, wherein the low-pass filter has a time constant that is set to vary in such a way that the higher the state of health detected by the state-of-health detecting means becomes the smaller the time constant becomes, and such that the higher the state of charge estimated by the state-of-charge estimating means becomes the smaller the time constant becomes, characterized by