Provided is a method for forming a cell aggregate for self-organization, whereby it is possible to discover conditions necessary for fabricating a cell aggregate from numerous cells (tens of thousands to hundreds of thousands) in vitro, and to realize complex higher-order structures of a liver, kidney, or the like or interactions thereof with other organs. A method for fabricating a cell aggregate in vitro, the method including culturing a mixture of at least a total cell count of 400,000 cells of any cell and/or tissue type and 100,000 to 400,000 mesenchymal cells and forming a cell aggregate having a size of at least 1 mm. A cell aggregate fabricated by the aforementioned method. A method for fabricating a three-dimensional tissue structure, the method including causing a cell aggregate fabricated by the aforementioned method to self-organize and forming a three-dimensional tissue structure endowed with a higher-order structure. A gelled culture support, the cross-section of which on the culturing side thereof being U-shaped or V-shaped.
