This study describes microleakage sensing based on swept-source optical coherence tomography (SS-OCT). With a handheld scanning probe, the SS-OCT system can provide portable real-time imaging for clinical diagnosis. Radiography is the traditional clinical imaging instrument used for dentistry; however, it does not provide good contrast images between filling material and the enamel of treated teeth with microleakage. The results of this study show that microleakage can be detected with oral probing using SS-OCT in vivo. The calculated microleakage length was 401 m and the width is 148 m, which is consistent with the related histological biopsy measurements. The diagnosis of microleakage in teeth could be useful for prevention of secondary caries in the clinical treatment plans developed in the field of oral medicine.