NOVELTY - The electronic control device (10A) comprises a switching element (20A) constituting a switching power supply which is operated using a power supply voltage (Vb) fed from a vehicle-mounted battery (11). When the power supply voltage is temporarily and abnormally lowered, charging electric charge of the output capacitor (22a) is flown back to the upstream power supply side by reverse conduction of the switching element, to suppress generation of undershoot. A reset circuit (80) prevents the microprocessor from being initialized by undershoot of the output capacitor.    USE - Vehicle-mounted electronic control device such as engine control devices, transmission control devices, etc.    ADVANTAGE - A vehicle-mounted electronic control device equipped with a constant voltage control device with low electric power consumption, which reduces a forward voltage drop of a switching element, suppresses an undershoot at the time when a power supply voltage is abnormally lowered, and generates an output voltage stabilized against fluctuations of the power supply voltage in a wide range, is provided. The minimum guaranteed voltage of the power supply voltage can be set smaller.    DETAILED DESCRIPTION - The vehicle-mounted electronic control device comprises a switching element constituting a switching power supply which is operated using a power supply voltage fed from a vehicle-mounted battery as an input voltage and is for stepping down the power supply voltage to generate a predetermined intermediate voltage. A continuously controlled type first constant voltage controlled power supply (30) or a same type second constant voltage controlled power supply (40) is connected in series to a later stage of the switching element and generates a first stabilization voltage (Vcc) or a second stabilization voltage (Vdd). A control circuit unit (50) includes a main control circuit section (54) which is fed from the second constant voltage controlled power supply and has at least a microprocessor (51), a program memory (52), and a RAM memory (53), or a subsidiary control circuit section (55) which is fed from the first constant voltage controlled power supply and is operative in cooperation with the main control circuit section. A downstream side coil (21a) serving as an inductive element is connected in series between the switching element and the first constant voltage controlled power supply or between the switching element and the second constant voltage controlled power supply. A flywheel diode (25) is connected between the downstream side coil and a ground circuit (GND) on the upstream side of the downstream side coil. An output capacitor is connected between the downstream side coil and the ground circuit on the downstream side of the downstream side coil. A value of an induced time constant which is a ratio between inductance of the downstream side coil and load resistance of an electric load fed from the switching element uses a value larger than a switching cycle at which switching control of the switching element is performed. The switching element is a positive (P) channel type or a negative (N) channel type FET. A circuit (70A) for suppressing reverse conduction is connected to a gate circuit of the FET. The reset circuit initializes the microprocessor when the intermediate voltage is lowered to less than a predetermined value. When the power supply voltage is temporarily and abnormally lowered, charging electric charge of the output capacitor is flown back to the upstream power supply side by reverse conduction of the switching element to suppress generation of undershoot. The reset circuit prevents the microprocessor from being initialized by undershoot of the output capacitor.    DESCRIPTION OF DRAWING(S) - The drawing shows the circuit block diagram of vehicle-mounted electronic control device. (Drawing includes non-English language text)    Vehicle-mounted electronic control device (10A)    Vehicle-mounted battery (11)    Switching element (20A)    Downstream side coil (21a)    Output capacitor (22a)    Flywheel diode (25)    Constant voltage controlled power supplies (30,40)    Control circuit unit (50)    Microprocessor (51)    Program memory (52)    RAM memory (53)    Main control circuit section (54)    Subsidiary control circuit section (55)    Circuit for suppressing reverse conduction (70A)    Reset circuit (80)    Ground circuit (GND)    Power supply voltage (Vb)    First stabilization voltage (Vcc)    Second stabilization voltage (Vdd)