A working machine has an oil groove communicating a pump chamber with a gear chamber, and a small amount of oil is delivered to the gear chamber when oil is discharged from the pump chamber. Thus, the oil adheres to the outer peripheral surface of a rotating pump shaft and is kept remaining between a pump case and the pump shaft. Therefore, the viscosity is increased and a pressure loss between the pump chamber and the gear chamber is increased. Accordingly, the airtightness can be maintained, and the stable and inexpensive oil pump mechanism requiring less man-hour can be realized without depending on the accuracy of a gap between the pump case and the pump shaft. Since oil is kept remaining on the outer peripheral surface of the pump shaft, it is possible to prevent the pump case from being worn out and to improve the durability of components.