An available input-output power estimating device for a secondary battery includes a parameter estimating section substituting a measured current value I and a measured voltage value V into an adaptive digital filter using an equivalent circuit model of the secondary battery for thereby collectively estimating values of parameters in the equivalent circuit model, an open circuit voltage calculator substituting I, V, and the estimated parameter values into the equivalent circuit model for calculating an open circuit voltage {circumflex over (V)}.sub.0, an available input power estimating section estimating an available input power P.sub.in of the secondary battery using components (a.sub.n,{circumflex over (b)}.sub.n), associated with a direct term in the equivalent circuit model, in the estimated parameters, the open circuit voltage estimate value {circumflex over (V)}.sub.0 and an upper limit voltage V.sub.max of the terminal voltage, and an available output power estimating section estimating an available output power P.sub.out for the secondary battery using (a.sub.n,{circumflex over (b)}.sub.n), {circumflex over (V)}.sub.0 and a lower limit voltage V.sub.min of the terminal voltage.