During a sound pressure measurement operational mode, when a distance from a vehicle and a corresponding required sound pressure level are specified, a control part causes a test sound to be outputted externally from an external sound output part by making a sound signal generation part generate a test sound signal as an external output sound signal. By doing this, the control part controls the signal level of the test sound signal as a result of sound pickup by a sound pickup part of the test sound outputted outside the vehicle, and thereby determines an adjustment value for the signal level of the test sound signal that can minimize the output sound volume, while the externally outputted sound propagated through the space outside the vehicle still satisfies required sound pressure conditions. The control part registers the determined adjustment value in a sound pressure adjustment table within a storage part.