PURPOSE:To easily penetrate a casing into hard ground by a method in which when the driving speed of the casing is lowered to less than a given speed, water jet is injected from the tip of the casing. CONSTITUTION:When the driving speed of a casing 6 is detected by a depth recorder and the driving speed of the casing 6 is lowered to less than a given speed due to the presence of a hard ground during the driving operation, water jet is injected from the nozzle 10 provided at the tip of the casing 6 by a signal from the depth recorder, whereby the casing 6 is penetrated through the hard ground. When the driving speed of the casing 6 is restored to the given speed, the injection from water jet is stopped by signal from the depth recorder.
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