We present a handheld device for the simple, stable, high-throughput, and viable circulating tumor cell (CTC) isolation with the whole blood processing capacity of 3.3ml/hr. The device uses the microfabricated tapered-slit filters having wider slit entrances and gradually narrower slit exits. Compared to the conventional straight-slit filters, the tapered-slit filters are capable to increase blood flowrate with the minimal cell stress. We verified the device's performance using the 4 different lung cancer cells at 5 different concentrations in the range of 5-100cells per 1ml of whole blood; thus achieving the average capture efficiency of 80.4% with the viability of 82.3%. Furthermore, the device captured 1 to 9 CTCs from the experiments using 5 different lung cancer patients' blood samples; thus, demonstrating the device's potential for clinical applications.