For estimation of a critical state of a performance under a working condition of a secondary cell, in consideration of an internal resistance of the cell, a variable data provider provides variable data on the performance, including a first data on a variant of the performance, a second data on a current of the cell, and an estimated third data on a dynamic characteristic of the internal resistance, a specified data provider provides specified data on the cell, including a fourth data on a permissible range of the variant under the prescribed working condition, and exact data on the dynamic characteristic, and a critical state estimator processes the variable and specified data for estimation of the critical state, and has an internal resistance setter to process the second and third data to set exact data as a function of second data defining the dynamic characteristic, and an estimating element to process the first, fourth, and exact data to estimate the critical state.