On the lunar surface, every human being would be exposed to galactic cosmic rays (GCRs) and their secondary products such as gamma rays and neutrons. For the human activity on the lunar surface in the future, it is important to estimate the effect of these particles on radiation doses. The annual ambient dose equivalent on the lunar surface was estimated on the basis of the latest observational data of GCRs. It is found that the annual ambient dose equivalent amount to about 570 mSv/yr during

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... e intermediate period between the maximum and the minimum phases of the solar activity. This amount of dose is mainly produced from primary components of GCRs heavier than proton and helium nuclei. The annual ambient dose equivalent due to iron nuclei during this period is about 130 mSv/yr, more than 20% of the total dose on the lunar surface. Moreover, the dose due to these neutrons among the secondary particles reaches 50 mSv/yr, suggesting that the dose due to neutrons must be considered from the viewpoint of the human activity on the lunar surface.