Apparatus for estimating charge state of secondary battery and method therefor are disclosed. The apparatus includes a control unit configured to estimate the state of charge of the secondary battery by repeatedly performing an algorithm of the Extended Kalman Filter by using a state equation that time-updates a state parameter including the state of charge of the secondary battery and a polarization voltage of the secondary battery, and an output equation that predicts the voltage of the secondary battery using an open circuit voltage according to the state of charge, the polarization voltage, and an internal resistance voltage generated by an internal resistance of the secondary battery, and the control unit increases a difference between state of charge noise and polarization voltage noise of the Extended Kalman Filter when the secondary battery becomes key-off state.