1. A notification sound control nit of an approaching vehicle audible system, generating a signal for a notification sound to be emitted from a sounding device provided in an electric vehicle in which at least part of driving force is produced by an electric motor to the outside of the electric vehicle, the notification sound control unit of an approaching vehicle audible system comprising: a first rotation speed calculation section that calculates a rotation speed, based on an accelerator opening degree signal among vehicle information signals of the electric vehicle; a second rotation speed calculation section that calculates a rotation speed, based on a vehicle speed signal among vehicle information signals of the electric vehicle; a rotation speed synthesis section that synthesizes a first rotation speed calculated by the first rotation speed calculation section and a second rotation speed calculated by the second rotation speed calculation section; a virtual engine rotation speed calculation section that applies filtering processing to a synthesis rotation speed synthesized by the rotation speed synthesis section so as to calculate a virtual engine rotation speed; and a notification sound signal generation processing section that changes a pitch and a volume of a sound element signal outputted from a sound element, based on the virtual engine rotation speed, so as to generate a notification sound signal.