This paper proposes a multi-port converter, which has multiple power sources with a buck converter or inverter connected in series, with an active power filter for a reduction of power conversion loss and circuit volume of an energy storage system. The proposed circuit consists of a multilevel converter, a series-active power filter, and an unfolder. In the proposed circuit, the number of inductors is reduced from four to two compared to the conventional circuit. Moreover, the proposed circuit reduces harmonic components of output voltage with a multilevel topology in order to reduce the volume of a filter inductor. In addition, a voltage control system is proposed for the converter to regulate the capacitor voltage of the active power filter. It is demonstrated that a 1-kW prototype circuit with Si-MOSFETs achieves a maximum efficiency of 99.3%. Furthermore, the capacitor voltage is regulated to constant even when the load power is stepped from no-load to full load.