In this study, a biosensor was successfully used to detect the impedance signals of various carcinoembryonic antigen (CEA) concentrations based on the developed sandwich-immunoassay method. CEA is a type of protein molecule that can be found in many different cells of the body, but is typically associated with certain tumors; therefore, CEA concentration detection is important for quantitative analysis. In this study, the gold nanoparticles (AuNPs) were introduced into a biosensor through the specific binding of antibodies, thus creating a bridge between two electrodes allowing electrons to move between them. When the concentration of the anti-CEA antibody-colloidal gold conjugates was diluted 30 times, the detectable limit for CEA concentration was 1 ng/mL with 1 μg/mL of anti-CEA antibody. Therefore, the developed biosensor is advantageous because it is effective with small sample volumes (25 μL), is a form of rapid quantitative measurement, and works with detection concentrations range in clinical diagnosis.