An apparatus for estimating state variables of a secondary battery as an estimation target performs so that a neural network unit studies a true value of a battery temperature, a temperature large value which is larger than the true value of the battery temperature by an approximating temperature sensor detection error, and a temperature small value which is smaller than the true value of the battery temperature by the approximating temperature sensor detection error. After completion of the learning of those values, the neural network unit inputs a battery temperature detected by the temperature sensor, and performs a neural network based calculation to calculate a SOC (state of charge) of the secondary battery. This can drastically increase the calculation accuracy of the SOC of the secondary battery.