An electric power conversion circuit system comprising: a primary side conversion circuit comprising a left arm and a right arm between a primary side positive bus and a primary side negative bus, wherein said left arm and said right arm are respectively composed of two switching transistors (S1, S2, S3, S4) connected in series, and a primary side winding of a transformer is connected between a connecting point of the two switching transistors (S1, S2) of said left arm and a connecting point of the two switching transistors (S3, S4) of said right arm; a secondary side conversion circuit comprising a left arm and a right arm between a secondary side positive bus and a secondary side negative bus, wherein said left arm and said right arm are respectively composed of two switching transistors (S5, S6, S7, S8) connected in series, and a secondary side winding of said transformer is connected between a connecting point of the two switching transistors (S5, S6) of said left arm and a connecting point of the two switching transistors (S7, S8) of said right arm; and a control circuit for controlling switching of said switching transistors of said primary side conversion circuit and said secondary side conversion circuit; wherein said control circuit is adapted to control the phase difference between the primary side conversion circuit and the secondary side conversion circuit and to control, when an output is at a relatively light loading, by changing of a duty command value of a transmitting side among said primary side conversion circuit and said secondary side conversion circuit, and by changing of a half-bridge phase difference between said left arm and said right arm of said primary side conversion circuit, and by changing of a half-bridge phase difference between said left arm and said right arm of said secondary side conversion circuit, characterized in that said control circuit is adapted to control changing of the duty command value of said transmitting side so as to be inversely proportional with respect to a phase difference φ between said primary side conversion circuit and said secondary side conversion circuit and changing of said half-bridge phase difference of said primary side conversion circuit using the duty command value of said secondary side conversion circuit and changing of said half-bridge phase difference of said secondary side conversion circuit using the duty command value of said primary side conversion circuit, so as to control the pulse widths of voltages applied across the transmitting side and across the receiving side of the transformer to be equal and to suppress a circulating current. a primary side conversion circuit comprising a left arm and a right arm between a primary side positive bus and a primary side negative bus, wherein said left arm and said right arm are respectively composed of two switching transistors (S1, S2, S3, S4) connected in series, and a primary side winding of a transformer is connected between a connecting point of the two switching transistors (S1, S2) of said left arm and a connecting point of the two switching transistors (S3, S4) of said right arm; a secondary side conversion circuit comprising a left arm and a right arm between a secondary side positive bus and a secondary side negative bus, wherein said left arm and said right arm are respectively composed of two switching transistors (S5, S6, S7, S8) connected in series, and a secondary side winding of said transformer is connected between a connecting point of the two switching transistors (S5, S6) of said left arm and a connecting point of the two switching transistors (S7, S8) of said right arm; and a control circuit for controlling switching of said switching transistors of said primary side conversion circuit and said secondary side conversion circuit; wherein said control circuit is adapted to control the phase difference between the primary side conversion circuit and the secondary side conversion circuit and to control, when an output is at a relatively light loading, by changing of a duty command value of a transmitting side among said primary side conversion circuit and said secondary side conversion circuit, and by changing of a half-bridge phase difference between said left arm and said right arm of said primary side conversion circuit, and by changing of a half-bridge phase difference between said left arm and said right arm of said secondary side conversion circuit, characterized in that said control circuit is adapted to control changing of the duty command value of said transmitting side so as to be inversely proportional with respect to a phase difference φ between said primary side conversion circuit and said secondary side conversion circuit and changing of said half-bridge phase difference of said primary side conversion circuit using the duty command value of said secondary side conversion circuit and changing of said half-bridge phase difference of said secondary side conversion circuit using the duty command value of said primary side conversion circuit, so as to control the pulse widths of voltages applied across the transmitting side and across the receiving side of the transformer to be equal and to suppress a circulating current.