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Leguminous plants are often cropped before main cropping in conservation agriculture based cropping systems. However, their quick decomposition and subsequent nitrogen (N) release may cause N leaching. In order to understand the effects of non-tillage cropping combined with hairy vetch (Vicia villosa Roth.) fallow as a cover crop on N balance considering water balance at sloping fields, an experiment was conducted on a sloping field at the Japan International Research Center for Agricultural Sciences (JIRCAS), Tropical Agriculture Research Front (TARF), Ishigaki Island, Okinawa Prefecture, Japan. Two fallow systems (natural and hairy vetch; HV) were combined with 2 soil tillage treatments (non-tillage and conventional tillage). Maize (Zea mays L.) was planted after the soil tillage treatment. Non-tillage after HV with half of the recommended fertilizer (vZ-1/2F) obtained similar grain yield (oven-dry basis; 5747 kg ha?1) to tillage after HV with half of the recommended fertilizer (vT-1/2F), non-tillage after natural fallow with recommended fertilizer (fZ-1F), and tillage after natural fallow with recommended fertilizer (fT-1F). It demonstrated a high effect of HV as substitute for chemical fertilizer for the succeeding crop. Leached nitrate-nitrogen (NO3-N) during the maize cropping was highest for vZ-1/2F (36.5 kg ha?1), followed by vT-1/2F (12.7 kg ha?1) and fT-1F (11.1 kg ha?1). A significant turnover of HV-N to the soil probably resulted in high N leaching for vZ-1/2F. Available N, calculated as fertilizer N + available rate x HV-N + Soil N, was almost equal to the sum of N uptake by maize and leached N. The residue mulch with non-tillage (vZ-1/2F) decreased water runoff, resulting in increased percolation. However, the NO3-N concentration in the percolated water was a much more important factor for N leaching calculation than the amount of percolation. This increased NO3-N concentration was probably due to decomposition of the HV biomass produced. However, the total N balance for maize cropping after HV fallow showed significant N surplus in the soil at the end of cropping. This must contribute to soil fertility improvement.