This invention relates to a denitrifying composition which is a material to be used for decomposing nitrates nitrogen in effluent by sulfur-oxidizing bacteria that consume sulfur and carbonate as nutrients and is characterized by containing particles of calcium carbonate dispersed in sulfur. Preferably, the composition contains 10 parts by weight of sulfur coexisting with 10-15 parts by weight of calcium carbonate and 1-3 parts by weight of a microporous substance. This denitrifying composition can be prepared by heating powder of calcium carbonate and sulfur thereby melting the sulfur, dispersing the powder of calcium carbonate in liquid sulfur and solidifying the dispersion by rapid cooling. The composition simultaneously contains nutrients and alkali source and hence enables denitrification to proceed stably without addition of other components.