A simple, label-free microfluidic cell purification method is presented for separation of cancer cells by exploiting difference in cell adhesion. To maximize the adhesion difference, three types of polymeric nanostructures (50nm pillars, 50nm perpendicular and 50nm parallel lines with respect to the direction of flow) were fabricated using UV-assisted capillary moulding and included inside a polydimethylsiloxane (PDMS) microfluidic channel bonded onto glass substrate. The adhesion force of human breast epithelial cells (MCF10A) and human breast carcinoma (MCF7) was measured independently by injecting each cell line into the microfluidic device followed by culture for a period of time (e.g., one, two, and three hours). Then, the cells bound to the floor of a microfluidic channel were detached by increasing the flow rate of medium in a stepwise fashion. It was found that the adhesion force of MCF10A was always higher than that of MCF cells regardless of culture time and surface nanotopography at all flow rates, resulting in a label-free detection and separation of cancer cells. For the cell types used in our study, the optimum separation was found for 2 hours culture on 50nm parallel line pattern followed by flow-induced detachment at a flow rate of $300{mu}l/min$.