The Nankai Trough is located at 200 km south from Shikoku Island, southwest Japan. It is elongated about 400 km from west to east. The Nankai trough is subduction zone between the Philippine see plate and the Eurasian plate. Many earthquakes have been occurring along this trough. Therefore The Nankai Trough is one of the major concentrations of strong seismic activity regions. The mud and sand deposits composed of marine and sub-marine sediments are found on the Oceanic crust as late Cenozoic trench sediments. Numerous geophysical surveys and conventional coring transects have been completed in this region. Several legs of Deep Sea Drilling Program (ODP) had performed the coring near The Nankai Trough. In this time, some drill core samples were collected from the bore hole of ODP Leg.190, Site 1174 preserved at the ODP center of Texas A&M University, USA. This bore hole reaches to almost Oceanic crust about 1100 m in depth. Therefore the borehole specimens are included of the trench-wedge deposits, Upper and Lower Shikoku Basin facies with decollement. The primary purpose of this study is to synthesize the results of clay mineralogical analysis for the core samples by means of X-ray diffraction. Electron probe microanalysis. Atomic absorption analysis. Differential thermal analysis and Transmission electron analysis. In particularly clay mineralogical characteristics of specimens collected from decollement were compared with the other specimens to discuss about the formation mechanism of decollement. In this study 38 specimens were collected from drilling cores, the drilling cores were divided into three facies, such as trench-wedge facies ( approximately 483.23 mbsf), Shikoku Basin facies (483.23-660.99 mbsf) and Lower Shikoku Basin facies (660.99- 1102.45 mbsf), respectively. They consist mainly of sub-marine mud with turbidities silt and sand, sub-marine mud with tuff and mud without tuff bed, respectively. The following minerals were measured in this site: chlorite, smectite, illite, kaolinite, quartz and plagioclase. The weight % of clay minerals was analyzed by the peak-intensity weighting method. Although the clay mineral assemblages show some diversity through the depth, smectite and illite are the most abundant clay minerals throughout the sampled section. Most abundant smectite was characteristically recognized in the specimens from decollement. Smectite is abundant in the Shikoku Basin facies and decreasing with depth. On the other hand, abundant of illite is increasing with depth. Depend on the CEC values obtained by Atomic absorption analysis, the exchange cations in the interlayer of smectite are recognized almost Ca ion with K and Mg ions.