1. A navigation system, comprising a navigation apparatus for carrying out guidance by outputting a voice message, and a server for communicating with the navigation apparatus, the navigation system further comprising: a detection part for detecting a location and a speed of the navigation apparatus; a voice message weight storage part for storing voice message weight association information, which associates a weight with each of multiple voice messages; a rule storage part for storing priority rule information denoting a rule for deciding a priority for each of the multiple voice messages; a priority decision part for deciding a priority for each of the multiple voice messages based on the priority rule information; and a voice output part for outputting the multiple voice messages from the navigation apparatus in a sequence conforming to the decided priority, wherein a rule denoted by the priority rule information is a rule for deciding a priority for each voice message based on the detected location and speed of the navigation apparatus, and the weight of each of the voice messages denoted by the voice message weight association information.