To provide a method for manufacturing a preform by which the shape retentive property of the preform consisting of a three-dimensional fiber structure can be easily secured by utilizing a sizing agent or a coupling agent contained in a fiber.

The method for manufacturing the preform comprises: a continuous fiber array step of arraying fiber bundles, the surface of each of which is treated by a reactive surface finishing agent, in one direction to form a fiber bundle layer, layering the fiber bundle layers one after another to form a biaxially-oriented, flat plate-shaped layered fiber group at the least; a thickness direction thread insertion step of inserting a thickness direction thread into the flat plate-shaped layered fiber group in the thickness direction as a connection thread; a pressurization/deformation step of pressurizing/deforming the layered fiber group by using a pressurizing means to form the three-dimensional fiber structure; and a heating treatment step of heat-treating the formed three-dimensional fiber structure.

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