To provide a high and low speed switching circuit for an earth drill which is capable of revolving an earth drill bucket at high speed in air.

Discharge oil of hydraulic pumps 41A, 41B is supplied to variable displacement hydraulic motors 43A, 43B via control valves 42A, 42B, and an earth drill bucket 34 is driven by hydraulic motors 43A, 43B. When a changeover switch 51 is turned to a high speed side, maximum displacement of the variable displacement hydraulic pumps 41A, 41B becomes a maximum value and displacement of the variable displacement motors 43A, 43B becomes a minimum value. At this time, the bucket 34 can be revolved at maximum revolving speed. When the changeover switch 51 is turned to a low speed side, maximum displacement of the variable displacement hydraulic pumps 41A, 41B and displacement of the variable displacement motors 43A, 43B are set to a minimum value and a maximum value, respectively. At this time, the bucket 34 can be revolved at minimum revolving speed.