A motor control unit that drives a motor through a motor driving circuit based on a calculated command value calculated by an MCU to which a power supply voltage is supplied from a power supply, comprising: a large-capacitance capacitor for charging and discharging electric charges of a motor driving voltage supplied to said motor driving circuit; a first FET with a parasitic diode, which is "ON/OFF"-controlled by said MCU and for separating said large-capacitance capacitor from said power supply voltage at a time of "OFF"; a second FET with a parasitic diode for an emergency blocking interposed between said power supply and said first FET; and a voltage detecting section to detect said power supply voltage and to input to said MCU; wherein a forward direction of said parasitic diode of said first FET and a forward direction of said parasitic diode of said second FET are inverse, characterised in that said power supply voltage is supplied to said MCU through a noise filter, and back flow preventing diodes are inserted in both a current supply path from said power supply voltage to said MCU and a current supply path from said large-capacitance capacitor to said MCU, wherein when said MCU recognizes as a limit drop in said power supply voltage that a voltage detected by said voltage detecting section is lower than a predetermined threshold, said first FET is turned- "OFF", said large-capacitance capacitor is separated from said power supply voltage without a back flow to said power supply voltage by said parasitic diode of said first FET, and a voltage is applied to said MCU from said large-capacitance capacitor so as to avoid a power supply reset of said MCU, wherein when said MCU recognizes a restoration of said power supply voltage, said first FET is turned-"ON", said large-capacitance capacitor is connected to said power supply voltage and a voltage is applies from said large-capacitance capacitor to said MCU, and wherein said power supply voltage is automatically supplied to said motor driving circuit and said large-capacitance capacitor by said parasitic diodes of said first and second FETs even when said first FET is not completely turned-"ON". a large-capacitance capacitor for charging and discharging electric charges of a motor driving voltage supplied to said motor driving circuit; a first FET with a parasitic diode, which is "ON/OFF"-controlled by said MCU and for separating said large-capacitance capacitor from said power supply voltage at a time of "OFF"; a second FET with a parasitic diode for an emergency blocking interposed between said power supply and said first FET; and a voltage detecting section to detect said power supply voltage and to input to said MCU; wherein a forward direction of said parasitic diode of said first FET and a forward direction of said parasitic diode of said second FET are inverse, characterised in that said power supply voltage is supplied to said MCU through a noise filter, and back flow preventing diodes are inserted in both a current supply path from said power supply voltage to said MCU and a current supply path from said large-capacitance capacitor to said MCU, wherein when said MCU recognizes as a limit drop in said power supply voltage that a voltage detected by said voltage detecting section is lower than a predetermined threshold, said first FET is turned- "OFF", said large-capacitance capacitor is separated from said power supply voltage without a back flow to said power supply voltage by said parasitic diode of said first FET, and a voltage is applied to said MCU from said large-capacitance capacitor so as to avoid a power supply reset of said MCU, wherein when said MCU recognizes a restoration of said power supply voltage, said first FET is turned-"ON", said large-capacitance capacitor is connected to said power supply voltage and a voltage is applies from said large-capacitance capacitor to said MCU, and wherein said power supply voltage is automatically supplied to said motor driving circuit and said large-capacitance capacitor by said parasitic diodes of said first and second FETs even when said first FET is not completely turned-"ON".