During the Observations at a Remote Island of Okinawa (ORION99) field campaign, HO2 radical and NOx concentrations were simultaneously measured with other important parameters relevant to photochemistry. The observed HO2 concentration levels, the relationship between HO2 and NOx, and the balance between NO and NO2 were well reproduced by model calculations based on the current photochemistry theory, indicating that the photochemistry taking place at the island was understandable within known chemistry. Thus the photochemical ozone production rate estimated from the calculated peroxy radical concentrations would be reliable. The isoprene peroxy radical + NO reaction contributed to ozone production by the similar degree to the HO2 + NO reaction because of high isoprene concentrations at ground level. The net ozone production rate during daytime was calculated to be +0.2--3.4 ppbv h-1 due to moderate NO concentrations (36--244 parts per trillion by volume (pptv)) there which were higher than the NO compensation point of ~30 pptv. Sensitivity studies showed that the ozone production rate was calculated within NOx-limited regime. Thus the ozone production rate would be relatively uniform throughout the boundary layer even if isoprene concentration had a steep vertical gradient. Up to ~20 ppbv of ozone would be photochemically produced in a day, suggesting that clear diurnal variations of ozone with daytime buildup of 16 ppbv observed for the air mass traveled over the island in 1996 could be explained by photochemistry.