1. A valve reference position-learning device for an internal combustion engine, comprising: a valve for regulating flow of exhaust gases or intake air of the engine; first reference position-learning means for learning a reference position of said valve at a start of the engine; second reference position-learning means for learning a reference position of said valve when predetermined learning conditions are satisfied after the start of the engine; reference position update means for updating the reference position based on the reference position learned by said first and second reference position-learning means; control means for controlling said valve using the updated reference position; and storage means for storing the reference position learned by said first reference position-learning means, wherein said reference position update means updates, when the reference position is learned by said first or second reference position-learning means, the reference position to the learned reference position, and updates, when learning of the reference position by said first reference position-learning means is not completed at the start of the engine, the reference position to the reference position stored in said storage means.