A state estimating device for a secondary battery, comprising: a detecting unit (20, 30, 40) for detecting a battery voltage, a battery current and a battery temperature of said secondary battery, thus obtaining detected values of a first state quantity, a second state quantity, and said battery temperature, wherein the first state quantity is one of said battery voltage and said battery current, and the second state quantity is the other one of said battery voltage and said battery current; a battery state estimating unit (110, 110A) configured to successively estimate, based on the detected value of said battery temperature and the detected value of the first state quantity, a charging rate of said secondary battery, an open-circuit voltage of said secondary battery and an estimated value of the second state quantity in accordance with a battery model equation, wherein an open circuit voltage is a potential difference between a positive electrode open-circuit potential and a negative electrode open-circuit potential; and a parameter estimating unit (130, 130A, 130B) configured to calculate an estimation error indicating a difference between the detected value and the estimated value of said second state quantity, and to estimate a predetermined parameter changing in accordance with a change in a state of said secondary battery, of a parameter group used for said battery model equation, based on said estimation error and one of said charging rate and said open-circuit voltage, wherein said battery state estimating unit (110, 110A) is configured to correct the positive electrode open-circuit potential and the negative electrode open-circuit potential by reflecting an estimation result of said predetermined parameter estimated by said parameter estimating unit (130, 130A, 130B) in said battery model equation, and to estimate said open-circuit voltage based on the corrected positive electrode open-circuit potential and the corrected negative electrode open-circuit potential. a detecting unit (20, 30, 40) for detecting a battery voltage, a battery current and a battery temperature of said secondary battery, thus obtaining detected values of a first state quantity, a second state quantity, and said battery temperature, wherein the first state quantity is one of said battery voltage and said battery current, and the second state quantity is the other one of said battery voltage and said battery current; a battery state estimating unit (110, 110A) configured to successively estimate, based on the detected value of said battery temperature and the detected value of the first state quantity, a charging rate of said secondary battery, an open-circuit voltage of said secondary battery and an estimated value of the second state quantity in accordance with a battery model equation, wherein an open circuit voltage is a potential difference between a positive electrode open-circuit potential and a negative electrode open-circuit potential; and a parameter estimating unit (130, 130A, 130B) configured to calculate an estimation error indicating a difference between the detected value and the estimated value of said second state quantity, and to estimate a predetermined parameter changing in accordance with a change in a state of said secondary battery, of a parameter group used for said battery model equation, based on said estimation error and one of said charging rate and said open-circuit voltage, wherein said battery state estimating unit (110, 110A) is configured to correct the positive electrode open-circuit potential and the negative electrode open-circuit potential by reflecting an estimation result of said predetermined parameter estimated by said parameter estimating unit (130, 130A, 130B) in said battery model equation, and to estimate said open-circuit voltage based on the corrected positive electrode open-circuit potential and the corrected negative electrode open-circuit potential.