This study investigated the change in denitrification rate in muddy tidal flats sediment by chemical and biological remediation. Column experiments were conducted to determine the dependence of denitrification rate on the redox potential. Filtered seawater was fed continuously to the pipe filled with sediment from the muddy tidal flats. An increase in the redox potential from -198 mV to 21 mV with bioturbation led to increases in the denitrification rate from 24.2 mg/m²/day to 29.8 mg/m²/day. The increase in denitrification was explained by the acceleration of mass transfer in the sediment. Redox-potential increased from -400 mV to -200 mV by the addition of 10 g calcium nitrate, acid volatile sulfide was below the detection limit to 20 cm depth, and denitrification rate was increased from 21.3 mg/m²/day to 36.4 mg/m²/day. It is considered that the increased nitrogen concentration led to an increase in microbial activity.