Our aim in this study was to demonstrate a stable immunoassay using the decrease of AC magnetic susceptibility without washing to separate bound and unbound magnetic markers. To achieve low noise in the immunoassay system, we examined the arrangement of the MR sensors, the calibration of the distance between the MR sensor and the sample, and the magnetic noise generated from the motor. The immunoassay system was improved based on these factors, and we detected mouse IgG conjugated polymer beads with anti-mouse IgG antibody-coated magnetic markers. The noise that resulted from the vibration of the MR sensor was decreased to about 1/6 by fixing the MR sensor on the excitation coils. The difference in the magnetic signal from each sample was also decreased from 40 % to 6% by calibrating the distance. Magnetic signal with a high signal-to-noise ratio (SN = 10 or more) was obtained by low-speed rotation (8 rpm) of the sample and the narrow band (5.3 Hz) of the lock-in amplifier demodulating the magnetic signal. The decrease of the AC magnetic susceptibility showed a strong correlation with the concentration of mouse IgG. Sensitivity of the immunoassay system could be estimated as about 4 fmol/ml in terms of molecular concentration. Immunoassay detection using AC magnetic susceptibility enabled a highly sensitive immunoassay without the conventional washing process.