The present study investigated the shear strain energy changes in a shear zone associated with interseismic plastic deformation and strike-slip earthquakes. A model of a very long strike-slip fault system was developed in which deep plastic flow is represented by an inelastic strain distribution. We derived analytical solutions for displacement and stress to estimate the changes in the shear strain energy. Applying this model to the San-in shear zone, Japan, we reproduced the observed surface velocity distribution by assuming a deep plastic flow with a horizontal thickness of T = 30 km, vertical thickness of 17 km, and strain rate of where 9 mm/year. We also found that the deep plastic flow roughly obeys a simple flow law. By using the stress changes induced by the deep plastic flow and the background stress estimated by a stress tensor inversion, we evaluated the changes in shear strain energy density in the shear zone. The energy at the seismogenic zone increases by J/m3/year when we assumed a differential stress of 100 MPa. We also evaluated the energy drop after two large earthquakes (MW 7.0 and 6.6). The estimated energy drop corresponded to an energy accumulation period of ~700 to ~1000 years, which is shorter than the period expected from geological observations.