A battery SOC estimation program comprising instructions which, when the program is executed by a computer, cause the computer to carry out a method of calculating state of charge of a battery mounted on a vehicle in a wireless transit system, the method comprising the following steps: set an equivalent circuit of the battery, the equivalent circuit being composed of three circuit elements connected in series, a first component resistance which is influenced by instantaneous current variation, a component circuit consisting of a capacitance and a second component resistance connected in parallel which is influenced by slow and excessive response, and an open circuit voltage of the equivalent circuit of the battery, receive measurement values of current and voltage of the battery, calculate, based on the measurement values, predetermined values of the ratio (k) of the second component resistance (R 2) to the first component resistance (R 1) and of the time constant (τ) as the product of the capacitance and the second component resistance, as constant values specific of the battery, and calculate (a) a value of the first component resistance from the detected variations in the measurement values of current and voltage of the battery, (b) a value of the second component resistance from the calculated value of the first component resistance and a predetermined value of said ratio (k), (c) a value of the capacitance from the calculated value of the second component resistance and a predetermined value of said time constant (τ), (d) a potential (V c) of the component circuit from the calculated values of the second component resistance and the capacitance, (e) a value of the open circuit voltage from the calculated potential of the component circuit, and (f) the SOC from the calculated value of the open circuit voltage. set an equivalent circuit of the battery, the equivalent circuit being composed of three circuit elements connected in series, a first component resistance which is influenced by instantaneous current variation, a component circuit consisting of a capacitance and a second component resistance connected in parallel which is influenced by slow and excessive response, and an open circuit voltage of the equivalent circuit of the battery, receive measurement values of current and voltage of the battery, calculate, based on the measurement values, predetermined values of the ratio (k) of the second component resistance (R 2) to the first component resistance (R 1) and of the time constant (τ) as the product of the capacitance and the second component resistance, as constant values specific of the battery, and calculate (a) a value of the first component resistance from the detected variations in the measurement values of current and voltage of the battery, (b) a value of the second component resistance from the calculated value of the first component resistance and a predetermined value of said ratio (k), (c) a value of the capacitance from the calculated value of the second component resistance and a predetermined value of said time constant (τ), (d) a potential (V c) of the component circuit from the calculated values of the second component resistance and the capacitance, (e) a value of the open circuit voltage from the calculated potential of the component circuit, and (f) the SOC from the calculated value of the open circuit voltage.