A state estimating device of a secondary battery, comprising: a detecting unit (20, 30, 40) for detecting a voltage, a current and a temperature of said secondary battery; and a battery state estimating unit for successively estimating a state quantity of said secondary battery according to a battery model equation based on at least one of the battery temperature (Tb) and the battery voltage (Vb), and the battery current (Ib) detected by said detecting unit, and characterized by: a storing unit for prestoring data relating to change characteristics of an initial state parameter value (Ran, Dsn) of said secondary battery with respect to changes in battery state in connection with a predetermined parameter (Ra, Ds) of parameters used in said battery model equation; and a change rate estimating unit (130, 200) for estimating a parameter change rate (gr, gd) being a ratio of a present parameter value with respect to said initial state parameter value in connection with said predetermined parameter, wherein said change rate estimating unit is adapted to perform parameter identification based on said battery model equation using said battery temperature, said battery voltage and said battery current detected by said detecting unit as well as said initial state parameter value corresponding to the present battery state based on the data read from said storing unit, and thereby to estimate said parameter change rate, and said battery model equation includes: a voltage equation expressed by an open-circuit voltage (U(θ)) being a function of a reaction-participating material concentration at a surface of an active material inside said secondary battery and an overvoltage (Ra(θ,T) x I) depending on the current, and a diffusion equation defining a distribution of said reaction-participating material concentration inside said active material. a detecting unit (20, 30, 40) for detecting a voltage, a current and a temperature of said secondary battery; and a battery state estimating unit for successively estimating a state quantity of said secondary battery according to a battery model equation based on at least one of the battery temperature (Tb) and the battery voltage (Vb), and the battery current (Ib) detected by said detecting unit, and a storing unit for prestoring data relating to change characteristics of an initial state parameter value (Ran, Dsn) of said secondary battery with respect to changes in battery state in connection with a predetermined parameter (Ra, Ds) of parameters used in said battery model equation; and a change rate estimating unit (130, 200) for estimating a parameter change rate (gr, gd) being a ratio of a present parameter value with respect to said initial state parameter value in connection with said predetermined parameter, wherein said change rate estimating unit is adapted to perform parameter identification based on said battery model equation using said battery temperature, said battery voltage and said battery current detected by said detecting unit as well as said initial state parameter value corresponding to the present battery state based on the data read from said storing unit, and thereby to estimate said parameter change rate, and said battery model equation includes: a voltage equation expressed by an open-circuit voltage (U(θ)) being a function of a reaction-participating material concentration at a surface of an active material inside said secondary battery and an overvoltage (Ra(θ,T) x I) depending on the current, and a diffusion equation defining a distribution of said reaction-participating material concentration inside said active material. a voltage equation expressed by an open-circuit voltage (U(θ)) being a function of a reaction-participating material concentration at a surface of an active material inside said secondary battery and an overvoltage (Ra(θ,T) x I) depending on the current, and a diffusion equation defining a distribution of said reaction-participating material concentration inside said active material.