A method of producing a cell mass by three-dimensional culture of primary cancer cells having proliferative ability and properties of handleability, versatility, and high-throughput performance, in which a tumor tissue is used as a starting material, proliferation of cells such as fibroblasts other than cancer cells is inhibited, and the cell mass includes primary cancer cells as a main component. The object is achieved by providing a method of producing a cell mass by three-dimensional culture of primary cancer cells using a tumor tissue, including: a three-dimensional culture step of culturing cells obtained from the tumor tissue in a medium containing a 5% v/v or less extracellular matrix on a substantially low-adhesive cell culture substrate.