To obtain a microorganism carrier excellent in durability, the settlement of microorganisms, lasting properties and waste water treatment efficiency, in the microorganism carrier formed by knitting synthetic fibers, by applying loop knitting to at least the single surface of a knitted fabric and pleating the knitted fabric.

In a microorganism carrier formed by knitting synthetic fibers, loop knitting 1 is applied to at least the single surface of a knitted fabric and pleating (tacks or pleats) 3 is formed to the knitted fabric. The loop knitting 1 of fibers is applied to at least the single surface of the knitted fabric of synthetic fibers because the adhesion and holdability of microorganisms are ensured and the surface area of the microorganism carrier is increased. The pleating 3 is formed in order to enhance the practical rigidity of the microorganism carrier, to prevent the shaking or mutual contact due to water streams, to increase the contact area with waste water to shorten a treatment time and to accumulate microorganisms on the interiors (cavities) of curls formed by pleats.