The authors conducted the study to demonstrate the nitrate (NO3-) transport and reduction process in the coastal groundwater of Seto Inland Sea, southern Japan. The results are summarized as follows: 1) NO3--N concentrations in the shallow groundwater were about 30mgL-1 in the midstream area of the catchment, whereas it decreased less than 2.0mgL-1 in the downstream area toward the groundwater flow direction. The inverse relationship between HCO3--C and NO3--N concentrations suggests biochemical denitrification process in the shallow groundwater. This is also supported by the low ratio of seawater contribution (<1.0%), and relatively low ORP in the groundwater of the downstream area; 2) The NO3--N concentrations in the downstream area were relatively higher in the High-flow season than that in the Low-flow season. This result indicates that the NO3--N discharge by shallow groundwater in the High-flow is larger than that in the Low-flow season; 3) NO3--N concentrations in deep groundwater were higher than that in shallow groundwater at the downstream area. This result suggests that leaching of organic matters from the upper sedimentary layer and groundwater recharge by the river water leads to supply of DOC to the shallow groundwater, and as a result it significantly increase denitrification process in shallow aquifer; 4) Based on these results, NO3--N discharge by groundwater from the study catchment is estimated to be about 5.0% of the annual nitrogen input from fertilizer application at orange groves.