To provide a three-dimensional hollow structure formed of an inorganic material exhibiting remarkable effect in high heat resistance or the like at low production cost by a relatively simple constitution, and to provide a method for effectively preventing the breaking or the like of a middle core.

A planar sheet material is formed so as to make a three-dimensional hollow structure, and an inorganic binder solution as an external addition agent consisting of metakaolin, a coagulant or the like is impregnated into the sheet material. Alternatively, slurry obtained by blending inorganic powder or the like as the main material with the above inorganic binder solution as an internal addition agent is shape-imparted to form a sheet material. Further, as for the delta core structure, in a state where a sheet material S is held between a receiving die 50 and a pressing die 60, the lower end edge of the pressing die 60 is moved from a position opposite to the upper end edge of either slope of press grooves 52 of the receiving die 50 to the lower end edge along the either slope, so that the sheet material S is bent to a cross-sectional shape forming an equilateral triangl shape to impart the shape thereto, and a large number of the shape-imparted ones are continuously arranged in a state of a straight line, and are therefor formed into a cross-sectional tooth-shaped.