Many water quality conservation measures have been implemented in agricultural areas to reduce pollutant loading. We investigated agricultural drainage canals in three paddy-field districts around Lake Biwa. In this paper, we focus on nitrogen (N) and organic carbon (C) released from the sediments in the drainage canals. We collected sediment cores and conducted an incubation experiment. We measured temporal change in concentrations of N and organic C in the overlying water of the sediments. As a result, we found that the nitrogen is released mainly as inorganic nitrogen (ammonium N). Ammonium N flux for each sediment sampled from St. 1, St. 2 and St. 3 was 24, 13, and 41 mg N/m²/d, respectively. Nitrogen was released from the sediments mainly in an inorganic state. The C/N ratio of the sediments was about 10 and almost constant during the incubation period. It is suggested that the fraction of organic decomposed in the sediments was small during the incubation experiment.