The purpose of this study was to evaluate gap formations under class V restoration using swept-source optical coherence tomography (SS-OCT). Wedge-shaped cavities were prepared on the buccal surface of 40 extracted premolar teeth at 2 locations; 1) cementoenamel junction (CEJ) with enamel and cementum margin and 2) root surfaces with cementum margin. The cavity was treated with Clearfil S 3 Bond, restored with Clearfil Majesty and polished with abrasive disks. The specimens were kept in water at 37°C for 24 hours and subjected to a thermocycling procedure. Gap formations at the tooth-restoration interface were measured with SS-OCT image and conventional dye leakage under a microscope. There was no effect of the locations of the cavity and the margins of the cavity on the gap formation. Therefore, a significant effect of the observational methods was observed. The gap formation was 0.89±0.48 mm with the SS-OCT, and the gap formation was 0.34±0.41 mm with the dye leakage. The observation with SS-OCT demonstrated a greater degree of gap formation than the observation with dye leakage.