An inputtable/outputtable power estimating device and method that can correctly estimate maximum inputtable and/or outputtable power of a secondary battery that can continue for a prescribed period of time. When the secondary battery is continuously charged or discharged for a prescribed period of time, the estimated value of a maximum possible charge and/or discharge current is estimated such that the terminal voltage of the secondary battery reaches a predetermined upper limit voltage or lower limit voltage. The inputtable and/or outputtable power is estimated as the power that can be used to charge the secondary battery or be discharged from it on the basis of the estimated maximum possible current(s) and the respective upper limit voltage or lower limit voltage.