A motor drive system comprising: a direct current power supply; a three-phase alternating current motor; an electric power converter disposed between the direct current power supply and the three-phase alternating current motor, the electric power converter including switching circuits, each of which corresponds to respective one of three phases of the three-phase alternating current motor, and a smoothing condenser, which is disposed electrically in parallel with the switching circuits; and a boost converter disposed between the electric power converter and the direct current power supply, the boost converter boosting a direct current voltage of the direct current power supply and supplying it to the electric power converter, an operating device comprising a proportional element, an integral element and a derivative element, the derivative element being configured as a bandpass filter, said operating device calculating a PID controlled variable corresponding to an electric current command value of the boost converter for maintaining an output voltage of the boost converter at a command value of an inter-terminal voltage VH of the smoothing condenser; a controlling device for controlling the output voltage of the boost converter on the basis of the calculated PID controlled variable; characterized by a judging device for judging whether or not suppression of loss associated with input/output of an electric power on the direct current power supply is to be prioritized over suppression of variation in the inter-terminal voltage VH by judging whether or not the variation in the inter-terminal voltage VH exceeds a predetermined limit; and a switching device for switching an operation mode when said operating device calculates the PID controlled value from a variation suppression mode in which a center frequency (fc) of the bandpass filter is maintained at a reference value (fcbase) to a loss suppression mode in which the center frequency (fc) is corrected to a lower frequency side with respect to the reference value (fcbase) if it is judged that the loss suppression is to be prioritized, in the loss suppression mode, the center frequency (fc) being corrected to a lower frequency (fccor) as the rotational speed of the three-phase alternating current motor decreases, and the center frequency being set in a band between an upper limit of electrical frequency (FBmtr, fmt) of the three-phase alternating current motor and a lower limit of switching frequency (FBinv, fcar) of the switching circuits of the electrical power converter. a direct current power supply; a three-phase alternating current motor; an electric power converter disposed between the direct current power supply and the three-phase alternating current motor, the electric power converter including switching circuits, each of which corresponds to respective one of three phases of the three-phase alternating current motor, and a smoothing condenser, which is disposed electrically in parallel with the switching circuits; and a boost converter disposed between the electric power converter and the direct current power supply, the boost converter boosting a direct current voltage of the direct current power supply and supplying it to the electric power converter,