PURPOSE: To sample the core of improvement soil easily by providing a rectangular bucket equipped with a cover capable of being opened and closed with a slide mechanism on the front thereof to a stirring wing of an excavation stirring shaft in a detachable manner, and providing wires for opening and closing the cover to both up and down of the cover on the front of the bucket.

CONSTITUTION: A guide having a U-shaped section is inserted in a stirring wind 12 right above an excavation wing 11, and a rectangular bucket is fixed to the U-shaped guide. As a slide construction, the front of the rectangular bucket 2 is pulled upward to open a cover thereof, and a front wire 24 is mounted to the upper part of the cover so that the cover can be pulled out. The rectangular bucket 2 is provided to an excavation stirring shaft, and the force-fit of the excavation stirring shaft is made in the ground after it is stirred with cement milk while keeping a state to lightly pull a rear wire 25 for closing the front cover. When the excavation stirring shaft is lowered up to a specific depth, the rear wire 25 is loosed, the front wire 24 is pulled to open the cover of the rectangular bucket 2, and when the excavation stirring shaft is revolved, a sample of required core is scooped in the rectangular bucket 2.