A mobility service supporting device includes a communication unit configured to be communicable with a vehicle-mounted device installed in a vehicle subjected to autonomous driving control and a portable terminal of a user of the vehicle and a controller configured to transmit, based on information about the position of the vehicle which is received from the vehicle-mounted device via the communication unit, notification for preventing the portable terminal from being left behind in the vehicle to the portable terminal of the user via the communication unit before the vehicle arrives at a destination of the user.