A motor drive apparatus comprising: a first drive circuit driving a first motor; and a voltage converter performing a voltage conversion between a power supply and said first drive circuit; a capacitor provided between said voltage converter and said first drive circuit; and a control device for controlling said first drive circuit and said voltage converter; wherein said voltage converter performs a voltage step-up operation of stepping up a power supply voltage to an arbitrary level and outputs the stepped-up voltage, and wherein said first drive circuit starts an electric power conversion for driving said first motor in powering mode after said voltage step-up operation is completed, characterized in that, said first motor is a motor starting or stopping an internal combustion engine, and said voltage converter starts said voltage step-up operation when an instruction to start said internal combustion engine is output, and the motor drive apparatus further comprises: target voltage determination means adapted to determine a target voltage of said stepped-up voltage based on the number of revolutions of said first motor; and voltage conversion control means (52, 54, 56) receiving the target voltage determined by said target voltage determination means for controlling said voltage converter to set said stepped-up voltage to said target voltage, wherein receiving said instruction to start said internal combustion engine, said voltage conversion control means (52, 54, 56) controls said voltage converter to obtain a predetermined stepped-up voltage that is necessary for starting said internal combustion engine, regardless of said determined target voltage, said predetermined stepped-up voltage being a maximum voltage of said motor drive apparatus, and said voltage conversion control means (52, 54, 56) determines a voltage step-up rate, which is the rate of change of output voltage of the voltage converter in voltage step-up operation, so that electric power required for said voltage step-up operation is within the electric power level that can be output from said power supply, and said power supply voltage is stepped up to said predetermined stepped-up voltage at said determined voltage step-up rate. a first drive circuit driving a first motor; and a voltage converter performing a voltage conversion between a power supply and said first drive circuit; a capacitor provided between said voltage converter and said first drive circuit; and a control device for controlling said first drive circuit and said voltage converter; wherein said voltage converter performs a voltage step-up operation of stepping up a power supply voltage to an arbitrary level and outputs the stepped-up voltage, and wherein said first drive circuit starts an electric power conversion for driving said first motor in powering mode after said voltage step-up operation is completed, characterized in that, said first motor is a motor starting or stopping an internal combustion engine, and said voltage converter starts said voltage step-up operation when an instruction to start said internal combustion engine is output, and the motor drive apparatus further comprises: target voltage determination means adapted to determine a target voltage of said stepped-up voltage based on the number of revolutions of said first motor; and voltage conversion control means (52, 54, 56) receiving the target voltage determined by said target voltage determination means for controlling said voltage converter to set said stepped-up voltage to said target voltage, wherein receiving said instruction to start said internal combustion engine, said voltage conversion control means (52, 54, 56) controls said voltage converter to obtain a predetermined stepped-up voltage that is necessary for starting said internal combustion engine, regardless of said determined target voltage, said predetermined stepped-up voltage being a maximum voltage of said motor drive apparatus, and said voltage conversion control means (52, 54, 56) determines a voltage step-up rate, which is the rate of change of output voltage of the voltage converter in voltage step-up operation, so that electric power required for said voltage step-up operation is within the electric power level that can be output from said power supply, and said power supply voltage is stepped up to said predetermined stepped-up voltage at said determined voltage step-up rate. target voltage determination means adapted to determine a target voltage of said stepped-up voltage based on the number of revolutions of said first motor; and voltage conversion control means (52, 54, 56) receiving the target voltage determined by said target voltage determination means for controlling said voltage converter to set said stepped-up voltage to said target voltage, wherein receiving said instruction to start said internal combustion engine, said voltage conversion control means (52, 54, 56) controls said voltage converter to obtain a predetermined stepped-up voltage that is necessary for starting said internal combustion engine, regardless of said determined target voltage, said predetermined stepped-up voltage being a maximum voltage of said motor drive apparatus, and said voltage conversion control means (52, 54, 56) determines a voltage step-up rate, which is the rate of change of output voltage of the voltage converter in voltage step-up operation, so that electric power required for said voltage step-up operation is within the electric power level that can be output from said power supply, and said power supply voltage is stepped up to said predetermined stepped-up voltage at said determined voltage step-up rate.