It was found that changes in the nitrogen concentration of leachate from the Osaka North Port sea based disposal site were closely related to the way in which dumping was carried out. The nitrogen concentration of the leachate was low due to the low nitrogen content and slow nitrogen dissolution rate of materials dumped previously in the landfill. The dumping of incinerator ash, noncombustible garbage, waterworks sludge and incinerated ash from sewage sludge were followed, and it was found that they caused a sharp increase in nitrogen concentration in the leachate. The main nitrogen form of leachate was NH4-N, and its concentration reached 50 mg/l after 6 years of landfilling. Successful nitrification treatment of leachate (more than 80% nitrification) was possible by using polyvinyl alcohol immobilized acclimated marine nitrifying sludge with an NH4-N loading rate of 2.9 mg-NH4-N/g-pellets/d. Low NO2-N was detected throughout the continuous nitrification experiments, so the rate limiting step in the nitrification treatment was revealed to be a nitrification step (NH4+-->NO2-). The addition of inorganic carbon to the test leachate enabled us to perform nitrification treatment even with a high NH4-N loading rate. Dolomite limestone was shown experimentally to be able to replace inorganic chemicals.