Disclosed is a novel cell mass derived from a cancer tissue, which can reflect the in vivo behavior of a cancer cell correctly. Also disclosed is a process for preparing the cell mass. Specifically disclosed is a cell mass derived from a cancer tissue, which is an separated product that is isolated from a cancer tissue obtained from an individual as a mass containing at least three cancer cells or a cultured product of the separated product and which can retain a proliferation ability in vitro. The cell mass derived from a cancer tissue is produced by, for example, a preparation process comprising the steps of: treating a pulverized product of a cancer tissue removed from a living body with an enzyme; and selecting and collecting a mass containing at least three cancer cells among from an enzymatic treatment product.