Monitoring temporal changes in antigen levels, such as disease-related markers in medical care or airborne allergens in environmental medicine, has been increasing in importance because it allows one to notice the level exceeding a threshold in real time, which helps the early detection or prevention of diseases. In this study, we have developed a reusable immunosensor that exploits surface plasmon resonance (SPR) and a pH-resistant protein, ORLA85, for rapid and repeated (semicontinuous) immunoassays. After the sensor development, a model experiment using mouse (mIgG) and anti-mouse (a-mIgG) antibodies was performed. In the experiment, the sensitivity of the developed SPR immunosensor was initially evaluated using various concentrations of a-mIgG. The limit of detection of the sensor for a-mIgG was 14.2 ng/mL (0.09 nM) and the dynamic range was 138–30000 ng/mL (0.86–187.5 nM). In 10 repeated measurements of a-mIgG, the sensor signals returned to the baseline after removing a-mIgG using an acidic solution with pH 1. The measurement time was 15 min and the coefficient of variation of the sensor outputs was 7.5%. These results indicated that the developed SPR immunosensor had potential for semicontinuous measurement, suggesting its application to monitoring systems for the early detection and prevention of diseases.