According to the present invention, a problem of closed systems, namely minimizing the number of electromagnetic valves required to control a plurality of flow paths, can be addressed, and thus a low-cost cell culture device can be achieved. In this flow-path control method for X number of flow paths satisfying X≤2N, the X number of flow paths are selected by using N number of valves to simultaneously and selectively control the opening and closing of the plurality of flow paths.