Abstract: A regenerative electric power storage system installed onboard a DC electric rail car comprises: a filter unit which removes high frequency components from the DC power supplied through the catenary of a pantograph; a storage unit for storing power; a bidirectional DC/DC converter which is electrically connected between the filter unit and the storage unit; a DC/DC filter unit to remove high frequency noise; a voltage detection unit and a control unit which controls the switching of the bidirectional DC/DC converter. Instability is presented in the remaining systems of the electric rail car.