A power conversion method of a power conversion device including a plurality of primary side ports disposed in a primary side circuit and a plurality of secondary side ports disposed in a secondary side circuit magnetically coupled to the primary side circuit with a transformer, the power conversion device adjusting transmission power transmitted between the primary side circuit and the secondary side circuit by changing a phase difference between switching of the primary side circuit and switching of the secondary side circuit, the power conversion method including: calculating a transmission efficiency setting an adjustment value of the phase difference to a value obtained by dividing the transmission efficiency by a specified efficiency; and setting the phase difference to a value obtained by dividing the calculated value by the adjustment value when the adjustment value is less than the specified value.