Twisted Magnetic Flux Bundle Models for Strong Solar Flares T. T. Ishii (Kyoto Univ.) and H. Kurokawa We studied what is the key process to trigger the major solar flares using observational data of the evolutions of active regions. In our previous studies, we have pointed out that the process of magnetic shear development is important for studying the above topic. We have constructed schematic models of emerging magnetic flux bundles to explain observed sunspot proper motions, and found that the emergence of twisted magnetic flux bundles relates to strong flare activities in several sunspot groups. During the current solar maximum (cycle 23), we studied the evolution of all the active regions that have produced at least one X-class flare and have been observed by the Solar and Heliospheric Observatory (SOHO) / Michelson Doppler Imager (MDI). We analyzed 38 active regions from 1996 through 2003. In this presentation, we report the common magnetic field geometry among flare-productive active regions and discuss the twisted magnetic flux bundle models of these regions.