A walk-behind, self-propelled working machine having a handle extending in a rearward and upward direction from a rear end of a body, an operation switching lever, and a drive switching lever. A speed adjusting rotary control for adjusting the driving speed of the working machine is provided in the vicinity of the drive switching lever with one end rotatably mounted to the side of the handle, being positioned near at hand for an operator. Even when the working machine is propelled, the speed adjusting rotary control can be operated with one hand while the drive switching lever is gripped with the handle with the other hand, thereby adjusting the driving speed of the working machine.