To enable soil lump to be smoothly and rapidly cracked by operating an agitator in a blending tank by applying light load and thereby to reduce construction work time and cost of treated soil.

Agitating blades 5 driven by a rotary shaft 3 in the blending tank 1 are provided at about two to four positions of an attaching part 4 and the agitating blade 5 is constituted of a main agitating blade 6 composed of a plate stretched from the attaching part 4 and an auxiliary agitating blade 7 joined to the main agitating blade 6. The main agitating blade 6 is attached to the attaching part 4 at a tilt angle of about 45°, and a shearing member 8, the cross section of which is composed of square lots and which is constituted of a frame-shaped part 8a and a stripe material part 8b extended in the horizontal direction at an intermediate position of the frame-shaped part 8a and other parts of which are vacant, is spreadingly provided at its top end part. Further, baffle plates 9 are attached onto an inner surface of a peripheral wall 1b of the blending tank 1 in a radial direction inwards.