A ground drilling machine comprises a controller for automatically controlling the injection of working liquid according to the output signal output as a function of the rotating condition of the pilot head 20. When the pilot head 20 is driven forward or backward, working liquid can be injected by switching the working liquid transfer valve 111 so long as the pilot head 20 is revolving in order to improve the digging efficiency, the soil delivering efficiency and the cooling efficiency of the machine. The injection of working liquid can be suspended when the direction of propelling the pilot head 20 is shifted while stopping the revolution of the pilot head 20 because no working liquid is required for delivering the dug soil and cooling the leading body. Thus, working liquid can be injected always at an optimal rate depending on the rotating condition of the pilot head 20 to suppress any waste of working liquid and reduce the construction cost.