Provided is a method in which the sediment in a water area where a methane-containing gas is apt to generate from the sediment can be stably inhibited from generating a methane-containing gas over a long period by laying a sediment modifier on the sediment. The method, which is for inhibiting the generation of a methane-containing gas by laying a sediment modifier on the sediment of a water area where a methane-containing gas generates, comprises laying a sediment modifier on the sediment of the water area, the sediment modifier comprising steelmaking slag that has a total iron content of 10 mass% or higher and having a content of granulated blast furnace slag of 0 mass% or higher but less than 20 mass%.本發明係提供在較容易從底質產生含甲烷氣體的水域中，藉由在底質上鋪設底質改質材，便可長期間安定地抑制來自底質產生含甲烷氣體的方法。本發明係藉由在會產生含甲烷氣體的水域底質上鋪設底質改質材，而抑制含甲烷氣體產生的方法，其係在水域底質上鋪設底質改質材，該底質改質材係含有全鐵含有量為10質量%以上的製鋼熔渣，且高爐水碎熔渣的含有量為0~未滿20質量%。
