To solve the problem that, to prevent the flotation of a manhole due to earthquake, etc., there are means for providing such a physical structure as to connect a plate to a manhole body as well as a method using a strainer filling pipe for filling hardening material into the outer periphery of the manhole to be solidified, but the hardening material depending on seepage pressure has no directivity and so its hardening range cannot be figured out.

The hardening material is sprayed and filled from a lateral spray nozzle 32 with pressure spray to give directivity to the filled hardening material. A rod is rotated to cause the jet flow of the hardening material to cut and agitate soil in a target ground, thus creating high-mixture-density and well-balanced hardened layers W which are arranged in parallel in the outer periphery of the manhole to be solidified.