To provide a bottom sediment-improving agent capable of improving habitat environment of algae, living things, etc., by improving the bottom sediment of closed sea bottom, moat bottom, etc., to decrease anaerobic bacteria and provide a method for improving the bottom sediment by using the above agent.

This bottom sediment-improving agent is obtained by forming magnesium hydroxide having 30-97% purity into granular, pellet-like or flake-like magnesium hydroxide having a hardness of 0.3-4.0 kg/granule and 2-9 mm particle diameter. The another objective method for improving the bottom sediment comprises spraying the bottom sediment improving agent on to organic substances-rich bottom sediment at a ratio of 100 to 1,000 g/m2 to carry out decomposing treatment of organic substances.