A method of squeezing a self-hardening material of stiff consistency into the ground through a jet pipe inserted into the ground when improving or strengthening the soft ground. While or after softening the ground to be treated by jetting high pressurized water from a high pressurized fluid jetting nozzle provided on the forward end of the jet pipe inserted into the soft ground, the self-hardening material of stiff consistency is squeezed into said softened ground to produce an improved part in the ground. A jet pipe having a jetting part projecting outward from the pipe may be used.