Automatic driving vehicles (ADV) are drawing attention all over the world. ADV contains many realtime sensors. In the future, people (including the driver) will enjoy BGM without attention to ambient conditions, however, it will be much better than the BGM corresponding to the surrounding situation in real time - rather than being properly chosen from existing music. The author proposes an approach of a “realtime musical composition system for automatic driving vehicles” which generates music in real time without using existing music, so we are free from the copyright issue. The realtime composition system can arrange/modify its generating musical factors/elements with realtime parameters such as sensor information in real time, so it is the best solution for “music in ADV”. This paper reports on the first prototype of realtime composition system for ADV - as collaborative research (2015–2017) with Toyota Central R&D Labs.