We present new structural data from SW Japan and discuss the mechanism of deformation during the opening of the Sea of Japan. We studied the Miocene basins at the southern margin of the Sea of Japan (northern coast of SW Japan) and of the Median Tectonic Line (MTL) by means of fault slip analysis to determine stress field directions during the opening of the Sea of Japan in early and middle Miocene time. On the southern margin, the stress field was extensional or transtensional with a NW‐SE trending extension direction. The MTL was a normal fault in Shikoku during early middle Miocene and probably a normal fault with a left‐lateral component in central Japan before having been strained by the collisions of the Tanzawa and Izu blocks with central Japan. We combine our results with published data for the Sea of Japan area and show that early and middle Miocene stress field directions are remarkably consistent on the eastern, southern, and western margins of the Sea of Japan with σ Hmax trending NE‐SW and σ Hmin trending NW‐SE. The stress regime is transpressional or transtensional on the eastern and western margin and almost purely extensional on the southern margin. This stress field distribution is in agreement with the model of the opening of the Sea of Japan in an extensional transfer zone between two N‐S right‐lateral strike‐slip zones proposed by Jolivet et al. (1991). Analogue modelings and kinematic reconstructions showed that this model accounts for 20° to 30° of clockwise rotation in SW Japan, while paleomagnetic rotations reached 40° to 50° during the opening. We present structural data showing that SW Japan was strained during the rotation and did not behave as a rigid block. Our observations suggest that rotations of small blocks may have occurred. We propose that SW Japan has been sheared between the right‐lateral strike‐slip zones which bound the Sea of Japan and that the missing 10° to 30° of rotation are to be found in its internal deformation. In our model, the MTL is a normal fault with a strike‐slip component rotating in a right‐lateral shear zone.