An internal resistance calculating device includes a charging unit charging a secondary battery, a voltage measuring unit measuring a both-end voltage value of the secondary battery, a capacitor holding a first voltage value predetermined by the both-end voltage value measured by the voltage measuring unit after charging is started, a capacitor holding, as a second voltage value, the both-end voltage value when charge current is changed for a predetermined current value or more within predetermined time after the both-end voltage value become the first voltage value, a current measuring unit measuring a first charge current value when the both-end voltage value become a threshold-value voltage and a second charge current value when the both-end voltage value become the second voltage value, and a μCOM calculating an internal resistance value of the secondary battery based on the first and second voltage values and the first and second charge current values.