The prototype of the autonomous lawn mower with a retrofittable driving control system was developed successfully. This retrofittable driving control system enabled to make an existing lawn mower autonomous, only by mounting sensors and actuators on it. It is widely known that lawn mowers with hydraulic mechanisms have hydraulic drive-derived problems such as operation delays of the hydraulic drive system and fluctuations in the reference position of the steering wheel rotation angle. Therefore, the autonomous driving control system solved these hydraulic drive-derived problems by, for example, estimating the reference position of the steering wheel rotation angle from the driving motions of the lawn mower. Furthermore, our autonomous driving control system can replicate the mowing work done by green-keepers with an autonomous lawn mower, by creating way points based on trajectory that an experienced green-keeper mowed the lawn on the fairway. The experimental results on the golf course showed the practical feasibility of the autonomous lawn mower with a retrofittable driving control system.