To minimize a ground space required, to increase the rigidity of an improved ground, and to shorten a construction period.

A propelling machine 4 having an excavator 4a is propelled from a start space 2 into a ground 1, then, a propelling tube 7 is propelled to an arrival space 9, and the propelling tube 7 is extracted from the ground 1 by a jack 10. A ground improver P, a high-pressure jet water, or both thereof are jetted from a jetting improvement device 5 toward the ground 1 to discharge mud while the pressure thereof is controlled by a mud discharge tube 19. The ground 1 is improved along the propelling direction of the propelling machine 4. After the ground improvement, a shape steel and a steel tube serving as reinforcement cores 11 are pushed into the ground by the jack.