We have developed the microfluidic chip to isolate gastric cancer cells from the peritoneal washing. Gastric cancer cells, which tend to be agglomerated in the peritoneal washing, are bigger than the other cells and the fragment of soft tissue. In this paper, we present a novel biochip to enrich low-concentrated cancer cells in the peritoneal washing by size-dependent cell filtration. This biochip has an advantage to avoid high-accuracy control of the flow rate because the cancer cells were filtered by the gap of the microchannel. Two-step exposure was carried out to achieve the precise 3D uneven channel for cell filtration. Furthermore by setting fins in the microchannel to assist the sheath flow, the capability to enrich the cancer cells was improved. As a result, we succeeded in double enrichment of the cancer cells in the quasi peritoneal washing.