To enable a user to know that an emergency vehicle is approaching without forcing a user to move his sight, moreover without drowning out a guidance sound.

An onboard navigation system 1 stores the sound loudness level in order to listen to the guidance sound of the route guidance set as the registered sound loudness level while considering the sound level of the pseudo notifying sound outputted from the emergency vehicle e.g. an ambulance car or fire engine etc. The onboard navigation system 1 reduces the sound level of the guidance sound to the registered sound level when the onboard navigation system determines that the emergency car is approaching and the guidance sound outputted into the vehicle chamber exceeds the registered sound level.