The purpose of our study is to demonstrate the effect of surface and groundwater interaction on nitrate attenuation in the coastal aquifer of a small alluvial fan catchment in southern Japan. NO3--N concentrations in the shallow groundwater decrease from the midstream area to the downstream area with groundwater flow. The inverse relationship between HCO3- -C and NO3--N suggests biochemical denitrification process in the downstream groundwater. This is also supported by relatively low ORP (Oxidation-Reduction-Potential) in the groundwater of the downstream area. Based on the results, it is suggested that mixing with domestic wastewater with high dissolved organic carbon (DOC) leads to supply of organic carbon to the groundwater. From the seasonal variation of NO3--N attenuation, it is suggested that the effect of denitrification on the downstream groundwater is smaller in the high-flow season than that in the lowflow because of large groundwater velocity. However, estimated flux of NO3--N attenuation is larger in the high-flow period because of high groundwater discharge. These results revealed the important role of interaction between surface water and groundwater in the alluvial fan on nitrate reduction in the coastal area.