An electric power converter (1A to 1D) for transmitting and receiving an electric power between an alternating current power source and a second battery or between a first battery and the second battery, the electric power converter (1A to 1D) comprising: a transformer having a primary winding (141a, 141b) and a secondary winding (142a, 142b); a rectifying circuit for rectifying an input voltage of the alternating current power source; a common bus bar for connecting the following members to the primary winding (141a, 141b) of the transformer: a first end of an output of the rectifying circuit, and a first electrode of the first battery; a first switch circuit (21 to 28) for selectively connecting the following members to the primary winding (141a, 141b) of the transformer: a second end of the output of the rectifying circuit, and a second electrode of the first battery, wherein the first switch circuit (21 to 28) is adapted to implement an electric power conversion; a second switch circuit (31, 32) for connecting the second battery to the secondary winding (142a, 142b) of the transformer, wherein the second switch circuit (31, 32) is adapted to implement the electric power conversion; and a controller for controlling the electric power of each of the alternating current power source, the first battery and the second battery by turning on and off a switch of the first switch circuit (21 to 28) and a switch of the second switch circuit (31, 32), wherein the second end of the output of the rectifying circuit is connected to the second electrode of the first battery by way of a diode, and wherein based on a ratio which is obtained by the following expression, the controller seeks for a time distribution of a time for supplying the electric power from the alternating current power source to the second battery by way of the transformer relative to a time for supplying the electric power from the second battery to the first battery by way of the transformer: ratio = electric power instruction X / electric power instruction X + electric power instruction Y where X denotes a charging electric power required for the second battery and Y denotes a charging electric power required for the first battery, wherein the controller monitors a voltage of the first battery, and according to the monitored voltage of the first battery, the controller adjusts the time distribution of the time for supplying the electric power from the alternating current power source to the second battery by way of the transformer relative to the time for supplying the electric power from the second battery to the first battery by way of the transformer, and according to the adjusted time distribution, the controller turns on and off the switch of the first switch circuit (21 to 28) and the switch of the second switch circuit (31, 32). a transformer having a primary winding (141a, 141b) and a secondary winding (142a, 142b); a rectifying circuit for rectifying an input voltage of the alternating current power source; a common bus bar for connecting the following members to the primary winding (141a, 141b) of the transformer: a first end of an output of the rectifying circuit, and a first electrode of the first battery; a first end of an output of the rectifying circuit, and a first electrode of the first battery; a first switch circuit (21 to 28) for selectively connecting the following members to the primary winding (141a, 141b) of the transformer: a second end of the output of the rectifying circuit, and a second electrode of the first battery, a second end of the output of the rectifying circuit, and a second electrode of the first battery, wherein the first switch circuit (21 to 28) is adapted to implement an electric power conversion; a second switch circuit (31, 32) for connecting the second battery to the secondary winding (142a, 142b) of the transformer, wherein the second switch circuit (31, 32) is adapted to implement the electric power conversion; and a controller for controlling the electric power of each of the alternating current power source, the first battery and the second battery by turning on and off a switch of the first switch circuit (21 to 28) and a switch of the second switch circuit (31, 32), wherein the second end of the output of the rectifying circuit is connected to the second electrode of the first battery by way of a diode, and wherein based on a ratio which is obtained by the following expression, the controller seeks for a time distribution of a time for supplying the electric power from the alternating current power source to the second battery by way of the transformer relative to a time for supplying the electric power from the second battery to the first battery by way of the transformer: ratio = electric power instruction X / electric power instruction X + electric power instruction Y where X denotes a charging electric power required for the second battery and Y denotes a charging electric power required for the first battery, wherein the controller monitors a voltage of the first battery, and according to the monitored voltage of the first battery, the controller adjusts the time distribution of the time for supplying the electric power from the alternating current power source to the second battery by way of the transformer relative to the time for supplying the electric power from the second battery to the first battery by way of the transformer, and according to the adjusted time distribution, the controller turns on and off the switch of the first switch circuit (21 to 28) and the switch of the second switch circuit (31, 32).