In some cases, it is difficult to accurately judge the position of the slip surface of a landslide caused by an earthquake from boring core and field observations. In this study, as a new technique for slip surface judgment, we applied a swelling pressure test based on the oven-dry method on core specimens and examined the resulting data characteristics to evaluate the effectiveness of the new slip surface judgment technique. The specimens used were boring cores collected from the field at locations of major slope disasters that occurred in Iwate prefecture during the 2008 Iwate-Miyagi Nairiku Earthquake. The test results show the following core characteristics. 1. The swelling pressure obtained from the oven-dry method is able to evaluate the swelling potential ability of landslide soil without being influenced by the initial water content of the specimens. 2. There is a high possibility that a slip surface may be present at a depth where swelling pressure suddenly increases to become significantly different from that at upper and lower levels. 3. The swelling pressure based on the oven-dry method can use as a quantitative index for the slip surface judgement. Through these and other observations, our study demonstrated the effectiveness of the new technique.