1. A control device of an engine comprising: an adjustment mechanism configured to adjust an intake air amount; and a controller configured to 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, the controller being configured to control the adjustment mechanism according to the first learning value, the controller being configured to save a second learning value in a non-volatile storage medium, the second learning value being equal to the first learning value, the controller being configured to correct the first learning value using the second learning value after the first learning value, is cleared.