A smooth current calculation apparatus, which calculates a smooth current of a secondary battery, includes a digital low-pass filter (22˜27) supplied with a measured digital current value generated by digitizing a measured current value of the secondary battery. The digital low-pass filter outputs a smooth digital current value, which is created by smoothing a time fluctuation of the measured digital current value. A current fluctuation level setting part (S1) compares a difference between the measured digital current value and the smooth digital current value with a threshold value, and sets a current fluctuation level based on a result of the comparison. A filter coefficient setting part (S2) sets a filter coefficient of the digital low-pass filter in accordance with the current fluctuation level.