To understand back-arc basin dynamics of the western Pacific, constraining the crustal type and rheology of the Ulleung Basin in the East Sea (Sea of Japan) is essential. We performed finite element modeling using a wide range of rheology to analyze the buckling structures observed in the Ulleung Basin with wavelengths of ~60-70 km and amplitudes of ~150-200 m. When a high Moho temperature (i.e., 570-640 °C) was adopted, both the oceanic and continental crustal models exhibited surface topographies and heat flow values similar to those observed in the region. Furthermore, the line force of the models was <1.5 TN/m, which is considerably lower than the known value from plate boundaries. The results indicate that the lithosphere beneath the Ulleung Basin is weak. Thus, we argue that the East Sea does not fully support far-field tectonic stress propagation from the Japan Trench to the Korean Peninsula.