The effects of understory dwarf bamboo (Sasa kurilensis) on the growth of overstory trees were studied in a dense secondary forest of Betula ermanii in northern Japan. Two plots were established in a dense Betula ermanii forest with Sasa in the understory, and Sasa was removed in one plot (removal plot) and Sasa was intact in the other plot (Sasa plot). Leaves of Betula ermanii in the Sasa plot have higher CO2 assimilation rates and larger area. Values of maximum quantum yield of PSII (Fv/Fm) are used as the index of photoinhibition. Decreases in Fv/Fm are observed neither in removal plot nor in Sasa plot. Excess excitation energy is dissipated as heat by de-epoxidation in xanthophyll cycle. The ratios of deepoxidation in xanthophyll cycle were high regardless of the presence/absence of Sasa. Even in the enzyme activities of the enzymatic antioxidant system, any difference was not seen.