To provide an on-vehicle type soil improving device capable of easily and economically obtaining stable improved soil even in an inclined place or an excavated place of different soil, concerning treatment of mud and muddy water produced in underground excavating and jacking work and underground continuous wall construction work.

A mixing-agitating-conveying two-shaft paddle screw conveyor for reacting pretreated mud and a solidifying agent to obtain improved soil in the on-vehicle type soil improving device is provided with its inclination angle adjusting device to adjust the inclination angle of the two-shaft paddle screw conveyor, thereby adjusting the amount of conveyance to improve mixing-agitating efficiency.