In this battery state estimating method, a current value and a terminal voltage value of a battery (B) are measured by means of a current detection unit (10) and a voltage detection unit (12) when a current is being charged or discharged, and on the basis of the current value and the terminal voltage value thus measured, at least one of charging rate, deterioration level or full charge quantity is calculated as a battery state. In the cases where a change of the measured current value per one second is a predetermined value or more, a current and a terminal voltage measured in a predetermined period of time after generation of the current change are not used, and on the basis of a current value and a terminal voltage value, which are measured during the charge/discharge period other than such period of time, at least one of the charging rate, the deterioration level or the full charge quantity of the battery (B) is calculated.