The style and explosivity of volcanic eruption are thought to be controlled by outgassing during magma ascent in a volcanic conduit. We have investigated the mechanism and rate of outgassing, which is based on laboratory experiments that simulate the magma ascent, coupled with microstructural observation of pyroclasts and lavas. The microstructure of bubbles in pyroclasts and fault texture in lavas indicate that shear deformation in flowing magma enhances outgassing. To quantify the rate of outgassing of the sheared magma, we performed torsional deformation experiments and measured the gas permeability of the sheared magma. This experiment demonstrated that shear deformation strongly increases the gas permeability. Moreover, we observed that magma ascending in the conduit can always obtain a gas permeability that is high enough to cause efficient outgassing if magma behaves as a Newtonian fluid. This efficient outgassing reduces the magma explosivity, which may make it difficult to induce explosive volcanism. Further, in situ X-ray radiographic and computed tomographic observations of magma deformation at high temperature show that shear localization in magma inhibited deformation and outgassing elsewhere. Thus, once shear localization starts during magma ascent, magma maintains its explosivity and causes explosive volcanism. On the other hand, non-explosive lava effusion occurs when magma is sheared well and efficient outgassing is experienced. To explore the cause of the variation in eruption style and explosivity, we need to understand elementary processes of the magma ascent such as magma rheology, vesiculation, outgassing, and fragmentation. In addition, it is important to understand how these processes affect one another, as indicated in the coupled effect of magma outgassing and rheology in this paper.