PURPOSE: To reduce the whole weight of a machine by a method in which the inner and outer tubes of the stirring shaft of a ground improver are shaped into a square cross section, and the periphery of the inner tube is contacted with the inner surface of the outer tube in such a way to make the stirring shaft in an expandible type.

CONSTITUTION: The stirring shaft 3 of a ground improver consists of an outer tube 7 connected to a vertical driver and an inner tube 6 inserted in a vertically movable manner into the outer tube 7, whose lower end is attached with shaft 3. The inner and outer tubes 6 and 7 have a square cross section, and the end of the tube 6 is contacted with the inner surface of the tube 7 and the lower end of the tube 7 is contacted with the periphery of the tube 6. A horizontal through hole 81 is formed traversely in the upper and lower ends of the tube 7 and in the upper end of the tube 6. A connecting pin 8 is thrust through the hole 81 detachably to fix the position of the tube 6. The expanding or contracting operation of the shaft 3 can thus be exactly attained.