A device for estimating a state of a secondary battery including first and second electrodes (12, 15) configured to include an active material containing a reactant material contributing to an electrochemical reaction and an ion conductor for conducting between said first and second electrodes said reactant material ionized, the device comprising: a voltage detector for detecting a voltage (Vb) of said battery between said first and second electrodes; a diffusion estimation unit for estimating a distribution in concentration of said reactant material in accordance with an active material diffusion model expression (M2a, M2b) defining said distribution in concentration of said reactant material in said active material, as based on a given boundary condition; an open circuit voltage estimation unit for estimating an open circuit voltage (U(θ)) between said first and second electrodes, as based on a concentration of said reactant material obtained at an interface of said active material with an electrolytic solution, as estimated by said diffusion estimation unit; a current estimation unit for estimating a density (I(t)) of a current of said secondary battery in accordance with a voltage-current relationship model expression (M3a-M3d) based on the electrochemical reaction and indicating a relationship between said open circuit voltage, an overvoltage (η 1 # - η 2 #) calculated in accordance with said density of said current of said secondary battery, a voltage drop (I(t)●Rd) caused in accordance with said density of said current of said secondary battery, and said voltage of said battery, wherein said voltage-current relationship model expression is such that the current estimation unit is configured to uniquely calculate said density of said current of said secondary battery without performing an iterative calculation, by substituting in said voltage-current relationship model expression said voltage of said battery detected by said voltage detector, said open circuit voltage estimated by said open circuit voltage estimation unit, and a parameter value of said secondary battery; and a boundary condition setting unit for calculating a reaction current density (j j Li) at said interface, as based on said density of said current of said battery estimated by said current estimation unit, and sets said boundary condition for said active material diffusion model expression for said interface in accordance with said reaction current density calculated. a voltage detector for detecting a voltage (Vb) of said battery between said first and second electrodes; a diffusion estimation unit for estimating a distribution in concentration of said reactant material in accordance with an active material diffusion model expression (M2a, M2b) defining said distribution in concentration of said reactant material in said active material, as based on a given boundary condition; an open circuit voltage estimation unit for estimating an open circuit voltage (U(θ)) between said first and second electrodes, as based on a concentration of said reactant material obtained at an interface of said active material with an electrolytic solution, as estimated by said diffusion estimation unit; a current estimation unit for estimating a density (I(t)) of a current of said secondary battery in accordance with a voltage-current relationship model expression (M3a-M3d) based on the electrochemical reaction and indicating a relationship between said open circuit voltage, an overvoltage (η 1 # - η 2 #) calculated in accordance with said density of said current of said secondary battery, a voltage drop (I(t)●Rd) caused in accordance with said density of said current of said secondary battery, and said voltage of said battery, wherein said voltage-current relationship model expression is such that the current estimation unit is configured to uniquely calculate said density of said current of said secondary battery without performing an iterative calculation, by substituting in said voltage-current relationship model expression said voltage of said battery detected by said voltage detector, said open circuit voltage estimated by said open circuit voltage estimation unit, and a parameter value of said secondary battery; and a boundary condition setting unit for calculating a reaction current density (j j Li) at said interface, as based on said density of said current of said battery estimated by said current estimation unit, and sets said boundary condition for said active material diffusion model expression for said interface in accordance with said reaction current density calculated.