A microorganism concentrator includes a water storage part that stores water sample therein and a filter that partitions the water storage part into a supply side and a drain side for the water sample. The microorganism concentrator concentrates microorganisms that are too large to pass through the filter on the supply side by moving the water sample on the supply side to the drain side of the water storage part. In the microorganism concentrator, the filter is arranged in such a way that no vertically downward force is applied to a surface of the filter on the supply side.