An electric motor drive control system, comprising: a DC power source; a converter configured to allow step up of an output voltage of said DC power source, for variably controlling the output voltage of said DC power source in accordance with a voltage command value ( VH#) and outputting to a DC power line; an inverter (20, 30) performing power conversion between an AC power driving an electric motor ( MG1, MG2) and a DC power on said DC power line, by a plurality of switching elements ( Q11-Q16, Q21-Q26), so that said electric motor ( MG1, MG2) operates in accordance with an operation command; and a controller setting said voltage command value of said converter; wherein said controller includes a first loss estimating unit estimating power loss ( Plb) at said DC power source, based on a first preset loss characteristic, which represents variation in said power loss ( Plb) at said DC power source with respect to at least one of rotation speed and torque of said electric motor ( MG1, MG2) and DC voltage on said DC power line, a second loss estimating unit estimating power loss ( Plcv) at said converter, based on a second preset loss characteristic, which represents variation in said power loss ( Plcv) with respect to at least one of rotation speed and torque of said electric motor ( MG1, MG2) and DC voltage on said DC power line, a third loss estimating unit ( 570, 570#) estimating power loss ( Pliv1, Pliv2) at said inverter ( 20, 30) based on a third preset loss characteristic which represents variation in said power loss ( Pliv1, Pliv2) at said inverter ( 20, 30) with respect to at least one of rotation speed and torque of said electric motor ( MG1, MG2) and DC voltage on said DC power line, and a voltage command value generating unit calculating a minimum necessary voltage ( VHmin) in correspondence with an induction voltage of said electric motor ( MG1, MG2), based on a state of operation of said electric motor ( MG1, MG2), establishing said DC voltage that minimizes total power loss ( Plt) including a sum of power losses estimated by said first to third loss estimating units, within a candidate voltage range not smaller than said minimum necessary voltage and not larger than an output upper limit voltage ( VHmax) of said converter, and setting said voltage command value in accordance with the established DC voltage. a DC power source; a converter configured to allow step up of an output voltage of said DC power source, for variably controlling the output voltage of said DC power source in accordance with a voltage command value ( VH#) and outputting to a DC power line; an inverter (20, 30) performing power conversion between an AC power driving an electric motor ( MG1, MG2) and a DC power on said DC power line, by a plurality of switching elements ( Q11-Q16, Q21-Q26), so that said electric motor ( MG1, MG2) operates in accordance with an operation command; and a controller setting said voltage command value of said converter; wherein said controller includes a first loss estimating unit estimating power loss ( Plb) at said DC power source, based on a first preset loss characteristic, which represents variation in said power loss ( Plb) at said DC power source with respect to at least one of rotation speed and torque of said electric motor ( MG1, MG2) and DC voltage on said DC power line, a second loss estimating unit estimating power loss ( Plcv) at said converter, based on a second preset loss characteristic, which represents variation in said power loss ( Plcv) with respect to at least one of rotation speed and torque of said electric motor ( MG1, MG2) and DC voltage on said DC power line, a third loss estimating unit ( 570, 570#) estimating power loss ( Pliv1, Pliv2) at said inverter ( 20, 30) based on a third preset loss characteristic which represents variation in said power loss ( Pliv1, Pliv2) at said inverter ( 20, 30) with respect to at least one of rotation speed and torque of said electric motor ( MG1, MG2) and DC voltage on said DC power line, and a voltage command value generating unit calculating a minimum necessary voltage ( VHmin) in correspondence with an induction voltage of said electric motor ( MG1, MG2), based on a state of operation of said electric motor ( MG1, MG2), establishing said DC voltage that minimizes total power loss ( Plt) including a sum of power losses estimated by said first to third loss estimating units, within a candidate voltage range not smaller than said minimum necessary voltage and not larger than an output upper limit voltage ( VHmax) of said converter, and setting said voltage command value in accordance with the established DC voltage.