The present invention addresses the problem of providing a method for detecting or separating/obtaining a circulating tumor cell (CTC), whereby it becomes possible to reliably and steadily detect or separate/obtain a CTC and a circulating tumor stem cell (CTSC) which are present in trace amounts in a biological circulating body fluid, such as blood and lymph fluid, even under the condition where the type of the tumor cell is not identified yet and the tumor cell is present in a trace amount in the biological circulating body fluid. The problem can be solved by a method for detecting or separating/obtaining a CTC and/or a CTSC in a biological circulating body fluid, said method comprising the following treatment steps (1) to (4): (1) a first step of pretreating a sample collected from the biological circulating body fluid to obtain a mononuclear cell phase; (2) a second step of providing a well plate that is prepared by injecting a liquid culture medium comprising a serum-free culture medium for circulating tu
