The present invention provides a device for biological wastewater treatment which prevents flotation and deformation of mesh tubes packed in a reactor tank as microorganism retaining materials. Device for biological water treatment 10 of the present invention comprises reactor tank 11; microorganism retaining media 12, which are installed in the tank; and air diffuser tubes 14, and wastewater distributor tube 16, both of which are installed under the microorganism retaining media. The microorganism retaining media comprise supporting media 22 and mesh tubes 24, which are installed on the supporting media. The supporting media are constituted in a manner that at least two the beams 26 are arranged on each supporting tier in parallel at certain intervals and the direction of the arrangement of the beams is alternately shifted by an angle of 90 degrees at every supporting tier. Also, the supporting tiers are piled in a manner that a space between two adjacent supporting tiers is enough for arranging mesh tubes on a supporting tier. The mesh tubes on each supporting tier are supported by beams of said supporting tier and arranged in parallel with beams on the adjacent tier.