Sika deer, Cervus nippon, population has recently increased in Japan. Damage to natural vegetation caused by deer becomes an urgent problem besides to forestry and agriculture. In Shikoku Is. few natural forests have remained after large-scale clearcutting and artificial regeneration during the latter half of the 20th century. Here we present the critical state of the natural deciduous forest around Mt. Sanbongui, containing the southernmost beech forest of Shikoku Is. Predominant species of the study area are Acer sieboldianum and Fagus crenata. Floor vegetation, consisting mainly of two dwarf bamboo species, which are primary forage for deer, had almost disappeared. We established six permanent plots in the spring of 2006, where we measured DBH and the size of feeding wounds of all living trees. We checked new dying trees and measured new wounds in every year from 2007 to 2011. 32% of all trees in number in six plots had feeding wounds in 2006. 9% of all trees died until 2011, and 65% of them had suffered bark-stripping. A. sieboldianum occupied 25% of all trees, and 55% of them had feeding wounds in 2006. 11% died until 2011, and 94% of them had suffered bark-stripping. The incidence of new wounds was 8% of all trees a year on average. These results show that this forest is rapidly declining by deer bark-stripping.