An engine control unit for automatically restarting an automatically-stopped engine in response to a restart requirement, in a vehicle including an engine speed sensor to detect the number (Ne) of revolutions of the engine, an injector to inject fuel into the engine, and a starter to apply torque to the engine via an engagement/disengagement mechanism for a gear, the engine control unit comprising: a restart controller to perform a first restart control if the number (Ne) of engine revolutions in response to the restart requirement is larger than a first number (Ne P1), the first restart control restarting the engine by the actuation of the injector without the actuation of the starter, and to keep the injector inactive until the number (Ne) of engine revolutions decreases to a second number (Ne P2) and perform a second restart control after the decrease in the number (Ne) of engine revolutions to the second number (Ne P2), if the number (Ne) of engine revolutions is equal to or smaller than the first number (Ne P1) in response to the restart requirement, the second restart control restarting the engine by the actuation of both the starter and the injector, the second number (Ne P2) being smaller than the first number (Ne P1); a failure determiner to determine a failure in the restart of the engine; and a failure mode controller to keep the injector inactive until the number (Ne) of engine revolutions decreases to the second number (Ne P2) and perform the second restart control after the decrease in the number (Ne) of engine revolutions to the second number (Ne P2), if the failure determiner determines the failure; characterized in that the failure determiner determines the failure through determination of satisfaction of at least one of two conditions: if the number (Ne) of engine revolutions decreases to a third number during the restart of the engine under the first restart control, the third number being smaller than the first number (Nepl) and larger than the second number or if the number (Ne) of engine revolutions is equal to or smaller than the first number (Nepi) after a certain number of restart strokes of the engine under the first restart control, said certain number being determined in advance depending on a preferred timing to terminate the first restart control. a restart controller to perform a first restart control if the number (Ne) of engine revolutions in response to the restart requirement is larger than a first number (Ne P1), the first restart control restarting the engine by the actuation of the injector without the actuation of the starter, and to keep the injector inactive until the number (Ne) of engine revolutions decreases to a second number (Ne P2) and perform a second restart control after the decrease in the number (Ne) of engine revolutions to the second number (Ne P2), if the number (Ne) of engine revolutions is equal to or smaller than the first number (Ne P1) in response to the restart requirement, the second restart control restarting the engine by the actuation of both the starter and the injector, the second number (Ne P2) being smaller than the first number (Ne P1); to perform a first restart control if the number (Ne) of engine revolutions in response to the restart requirement is larger than a first number (Ne P1), the first restart control restarting the engine by the actuation of the injector without the actuation of the starter, and to keep the injector inactive until the number (Ne) of engine revolutions decreases to a second number (Ne P2) and perform a second restart control after the decrease in the number (Ne) of engine revolutions to the second number (Ne P2), if the number (Ne) of engine revolutions is equal to or smaller than the first number (Ne P1) in response to the restart requirement, the second restart control restarting the engine by the actuation of both the starter and the injector, the second number (Ne P2) being smaller than the first number (Ne P1); a failure determiner to determine a failure in the restart of the engine; and a failure mode controller to keep the injector inactive until the number (Ne) of engine revolutions decreases to the second number (Ne P2) and perform the second restart control after the decrease in the number (Ne) of engine revolutions to the second number (Ne P2), if the failure determiner determines the failure; if the number (Ne) of engine revolutions decreases to a third number during the restart of the engine under the first restart control, the third number being smaller than the first number (Nepl) and larger than the second number or if the number (Ne) of engine revolutions is equal to or smaller than the first number (Nepi) after a certain number of restart strokes of the engine under the first restart control, said certain number being determined in advance depending on a preferred timing to terminate the first restart control.