1. A control device for an engine, the control device comprising: an adjustment mechanism configured to adjust an intake air amount; and an ECU configured to (a) save a first learning value in a volatile storage medium, the first learning value being obtained by learning an operation amount of the adjustment mechanism when the engine is idle, (b) control the adjustment mechanism according to the first learning value, (c) save a second learning value in a non-volatile storage medium, the second learning value being equal to the first learning value, and (d) correct the first learning value using the second learning value after the first learning value is reset to an initial value, (e) wherein the ECU is configured to correct the first learning value using the second learning value after the first learning value is reset to the initial value and when: the operation amount of the adjustment mechanism is an operation amount corresponding to an initial value of the first learning value, and an output torque of the engine is smaller than a predetermined value, and wherein when a power supply of the ECU is stopped and the first learning value saved in the volatile storage medium is reset to the initial value, the ECU is configured to restore from the non-volatile memory, the second learning value being equal to the first learning value before being reset to the initial value.