To make guidance voice easy to hear and understand for navigation user at all times regardless of its audio volume by interlocking the volume of the guidance voice of navigation to the audio volume in a navigation technique.

A guidance voice reproducing part A2 reproduces and outputs guidance voice. While the guidance voice reproducing part A2 is reproducing and outputting guidance voice, an attenuation part A3 attenuates audio voice from an audio reproducing device A1. A volume detection part A4 detects a volume value of audio voice from the audio reproducing device A1. An amplification part A5 amplifies guidance voice to a preset and prescribed amplification level. A determination changeover part 51 raises the amplification level in the case where a volume ratio of guidance voice to the volume value of audio voice detected by the volume detection part A4 is less than a prescribed reference value.