To provide a treatment method for an organic substance, or the like, in a supercritical state, or the like, capable of stable carrying out a reaction of the organic substance, or the like, and decomposition of a hardly decomposable substance by carrying out the chemical reaction in the supercritical state or a subcritical state near a first time even by a plurality of times, and an apparatus therefor.

In a reaction engine, a fluid vapor with the substance to be reacted introduced into a cylinder 3 is compressed by operating a compression plunger 4, and after the chemical reaction of the substance to be reacted by this compression is completed, the compression plunger 4 is operated in a reverse direction to reduce the temperature and the pressure of the fluid vapor, and an intake/exhaust stroke is periodically carried out by removing out the fluid containing the obtained product from the cylinder and blowing-in the new fluid vapor into the cylinder. The reaction method for the organic reactant, or the like, by a supercritical fluid or a subcritical fluid is characterized in that driving of an intake/exhaust valve in this reaction engine 2 is temporarily stopped to repeatedly operate the compression and expansion against the introduced fluid vapor by a plurality of times.

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