Cancer-derived small extracellular vesicles (sEVs) in body fluids hold promise as biomarkers for cancer diagnosis. For sEV-based liquid biopsy, isolation of sEVs with a high-purity and cancer-sEV detection with an extremely high sensitivity are essential because body fluids include much higher density of normal-cell-derived sEVs and other biomolecules and bioparticles. Here, we propose an isolation-analysis-integrated cancer-diagnosis platform based on dielectrophoresis(DEP)-ELISA technique which enables a three orders of magnitude higher sensitivity over conventional ELISA method and direct cancer diagnosis from blood plasma with high accuracy. The limit of detection (LOD) for sEVs in human plasma was as low as 10 sEVs/mL without a time-consuming and low-yield sEV isolation and purification process. The capability of this platform was validated by monitoring mice with cancer cell inoculation and assessing the effect of cancer-sEV-inhibiting drug. Using the developed sEV-based liquid biopsy, we diagnosed clinical samples from healthy donors (N = 39) and cancer patients (N = 90). The diagnostic accuracy was 94.2%, 98.6%, and 91.3% for breast, colon, and lung cancers, respectively. This integrated sEV isolation and analysis platform could be applied for high-sensitivity biomarker profiling and sEV-based liquid biopsy.