An object is to provide a method for detecting or separating/obtaining CTC, which is capable of reliably and stably detecting or separating/obtaining a circulating tumor cell and a circulating tumor stem cell present in trace amounts in a biological circulating fluid such as blood or lymph, even in the state where the cancer type of the tumor cells cannot be determined yet and the state where the tumor cells are present in trace amounts in the biological circulating fluid. The object is attained by a method for detecting or separating/obtaining CTC and/or CTSC in a biological circulating fluid, comprising the following treatment steps (1) to (4): (1) a first step of pretreating a sample from the biological circulating fluid to obtain a mononuclear cell phase; (2) a second step of providing a well plate in which a culture medium consisting of a serum-free cell growth medium for circulating tumor cell and/or circulating tumor stem cell has been injected, and seeding thereto the mononuclear cells obtained in the
