A main conversion device for an electric motor car, the main conversion device comprising: a first power converter that is connected with a circuit with an overhead line through which an AC current flows, and an intermediate link that is a circuit through which a DC current flows, the first power converter bi-directionally converting AC power and DC power; a second power converter that is connected with the intermediate link, and a circuit with an electric motor through which AC current flows, the second power converter bi-directionally converting the DC power and the AC power; an auxiliary power-supply device which is configured to receive power from the intermediate link, and which is configured to supply the power to a load; said main conversion device characterised in that it further comprises: a regenerative power calculator that is configured to calculate regenerative power to be generated by the electric motor based on the value indicating the acceleration condition of the vehicle and the value in accordance with the vehicle speed; an auxiliary source power calculator that calculates the auxiliary source power based on a voltage at the intermediate link and a current flowing in the auxiliary power-supply device, the auxiliary source power being necessary power for the auxiliary power-supply device to supply the power to the load; and an operation controller (120, 220, 320, 420, 520) that is configured to deactivate the first power converter when surplus power obtained by subtraction of the auxiliary source power from the regenerative power is larger than a restriction value for the regenerative power flowing to the overhead line, and to operate the first power converter when the surplus power is equal to or smaller than a threshold that is equal to or smaller than the restriction value. a first power converter that is connected with a circuit with an overhead line through which an AC current flows, and an intermediate link that is a circuit through which a DC current flows, the first power converter bi-directionally converting AC power and DC power; a second power converter that is connected with the intermediate link, and a circuit with an electric motor through which AC current flows, the second power converter bi-directionally converting the DC power and the AC power; an auxiliary power-supply device which is configured to receive power from the intermediate link, and which is configured to supply the power to a load; said main conversion device characterised in that it further comprises: a regenerative power calculator that is configured to calculate regenerative power to be generated by the electric motor based on the value indicating the acceleration condition of the vehicle and the value in accordance with the vehicle speed; an auxiliary source power calculator that calculates the auxiliary source power based on a voltage at the intermediate link and a current flowing in the auxiliary power-supply device, the auxiliary source power being necessary power for the auxiliary power-supply device to supply the power to the load; and an operation controller (120, 220, 320, 420, 520) that is configured to deactivate the first power converter when surplus power obtained by subtraction of the auxiliary source power from the regenerative power is larger than a restriction value for the regenerative power flowing to the overhead line, and to operate the first power converter when the surplus power is equal to or smaller than a threshold that is equal to or smaller than the restriction value. a regenerative power calculator that is configured to calculate regenerative power to be generated by the electric motor based on the value indicating the acceleration condition of the vehicle and the value in accordance with the vehicle speed; an auxiliary source power calculator that calculates the auxiliary source power based on a voltage at the intermediate link and a current flowing in the auxiliary power-supply device, the auxiliary source power being necessary power for the auxiliary power-supply device to supply the power to the load; and an operation controller (120, 220, 320, 420, 520) that is configured to deactivate the first power converter when surplus power obtained by subtraction of the auxiliary source power from the regenerative power is larger than a restriction value for the regenerative power flowing to the overhead line, and to operate the first power converter when the surplus power is equal to or smaller than a threshold that is equal to or smaller than the restriction value.