To reveal what is the key agent for strong flare activity, we studied the evolution of a large sunspot group, NOAA 4201, and its flare activity. We analyzed high-resolution H alpha images obtained with the Domeless Solar Telescope at Hida Observatory, and Kitt Peak full disk magnetograms. We examined the sunspot proper motions and evolutionary changes of H alpha fine structures and magnetic fields in this active region, and constructed a schematic model of emerging twisted magnetic-flux bundles. We also found that the occurrence of high flare activity was restricted to the rapidly emerging region of the twisted flux bundle. This fact suggests that the emergence of a twisted emerging flux bundle should be the key to high flare productivity of the sunspot group.