An apparatus for estimating a state of a secondary battery (SB), including: a charging unit electrically connected to two terminals of the SB, the charging unit charging the SB, a voltage measuring unit electrically connected to and measuring a voltage across the two terminals of the SB, a time measuring unit for: receiving a voltage value of the SB from the voltage measuring unit, and during charging of the SB, in a predefined voltage range, measuring a charge time of the SB corresponding to the voltage range multiple times, and a control unit for: transmitting/receiving an electrical signal to/from the charging unit, the voltage measuring unit, and the time measuring unit, calculating a ratio of a first charge time and a second charge time measured by the time measuring unit sequentially at a time interval, and estimating a state of the SB.