To provide an on-board system and a navigation device for a vehicle capable of effectively conveying the message to rear seat passengers who cannot hear due to wearing headphones even if spoken to from the front seat.

The navigation device is equipped with a rear seat display means which is mounted in a cabin visibly from the rear seat of the vehicle, and televises contents information such as movies, and a rear seat headphone outputting the sound. The navigation device is also equipped with a message storage means which stores message information conveyed from a driver or a front seat passenger of the vehicle to the rear seat passengers, and an operation input means disposed near the driver's seat of the vehicle. Moreover, the navigation device is equipped with an interruption control means which interruptively outputs the message information to at least either the rear seat display means during televising the contents information or the rear seat headphones during outputting the sound.