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; storage means for storing the reference position learned by said first reference position-learning means, as an existing learned value; second reference position-learning means for learning the reference position of said valve when predetermined learning conditions are satisfied after the start of the engine; reference position update means for updating, every time the reference position is learned by said first or second reference position-learning means, the reference position to the learned reference position, and when learning of the reference position by said first reference position-learning means is not completed at the start of the engine, updating the reference position to the existing learned value stored in said storage means; and control means for controlling said valve using the reference position.