To provide a method and an apparatus for subjecting a solid-containing object to be treated to continuous sub-critical water decomposition, in each of which a decomposition reaction of the object to be treated can be controlled, the scale of each of which can be enlarged, and in each of which the equipment cost can be kept low and desired useful substances can be produced selectively in high yield.

The object to be treated is crushed into particles in advance, which are then mixed with water to prepare slurry. The slurry is sent to a compressing means 1 through a pipe and compressed. The compressed slurry is sent to a heating means 2, heated and kept in a sub-critical state. The slurry of the sub-critical state is introduced into the bottom of a reaction vessel 3 through an inlet 8. A fixed bed, a fluidized bed and a sub-critical water dissolution layer are formed in the reaction vessel 3 in this order from the bottom. The sub-critical water dissolution layer is withdrawn through any of outlets 101-106 arranged in the upper part and side part of the reaction vessel 3 to adjust the residence time of sub-critical water and adjust the reaction time of the object which is to be treated and is subjected to the sub-critical water decomposition.

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