To intensify and improve the weak ground without impairing the surrounding environment, and also to effectively and strongly press-fit a friction joint pile.

After an auger screw 40 provided with a drilling machine 4 that has been juxtaposed together with a press-fitting machine 3 to the leader 2 of a pile driver or the like has been screwed into the weak ground, it is drawn out from the ground to form a screwed hole. A friction joint pile 30 that has been set in the press-fitting machine 3 is screwed in so as to be uprightly erected in a screwing hole H, and also a solidifying agent 6 is arranged around the friction joint pile 30. While utilizing the reaction force of the auger screw 40 by screwing the auger screw 40 in the vicinity of the screwing hole H, the press-fitting of the friction joint pile 30 is carried out by the press-fitting machine 3. Thus, the empty spaces caused between the joints of the friction joint pile 30 are packed and filled with the solidifying agent 6, so that the weak ground can be improved.