PURPOSE: To provide a novel biological nitration and denitrification method and apparatus capable of permitting org. sewage containing nitrogen to pass at a high filtering speed, reduced in the rising of filtering resistance and capable of highly purifying said sewage.

CONSTITUTION: An org. sewage supply part 4 and an oxygen-containing gas supply part 3 are provided to the intermediate part of a fixed bed 2 packed with an elastic porous granular solid in the height direction of said bed 2 and org. sewage 4 flows in the fixed bed 2 as a descending stream. Nitrifying bacteria nitrating NH3-N are immobilized on the part above the oxygen-containing gas supply part 3 of the fixed bed 2 and denitrifying bacteria 7 are immobilized on the part under the supply part 3 and a part of the outflow water from the bottom part of the fixed bed is supplied to the upper part of the fixed bed to be circulated as a descending stream.