PROBLEM TO BE SOLVED: To provide a high-pressure injection agitation method capable of performing construction at a water cement ratio of 0.8-0.6, preferably, at a water cement ratio of up to 0.5.SOLUTION: There is provided a high-pressure injection agitation method using cement slurry having a low water cement ratio, the high-pressure injection agitation method being characterized in that an additive to be added to the cement slurry consists of a carboxylic acid-based polymer obtained by combining and mixing a low molecular weight polymer and a high molecular weight polymer together so that a molecular weight distribution has two peaks, and when the cement slurry has a water cement ratio of 0.6 by adding a 0.5-3.0 wt% of the additive to a cement-based material as a hardener, the low molecular weight polymer and high molecular weight polymer are mixed so that the cement slurry has a starting fluidity of 10 seconds or less and a fluidity of 10.5 seconds or less three hours later by the JSCE-F521 standard (Japa
