Water cleaning system, water cleaning method, startup method for water cleaning system, and water cleaning unit

A water cleaning method comprising:

• installing, in water, a first bacteria inhabitable portion made of a soil material that is inhabitable by obligatory anaerobic bacteria and preliminarily pressed into a massive shape, forming, in water, a second bacteria inhabitable portion that is adjacent to the first bacteria inhabitable portion, has a particulate carrier, and is inhabitable by facultative anaerobic bacteria, a third bacteria inhabitable portion that is adjacent to the second bacteria inhabitable portion, has a particulate carrier, and is inhabitable by aerobic bacteria, and an anaerobic space that has an anaerobic environment and links with said first bacteria inhabitable portion, locating, at a position linking with said third bacteria inhabitable portion, an aerobic region including organic matter and oxygen, and linking the aerobic region and said anaerobic space, and
• forming an obligatory anaerobic layer by multiplying obligatory anaerobic bacteria in said first bacteria inhabitable portion, forming a facultative anaerobic layer by multiplying facultative anaerobic bacteria in said second bacteria inhabitable portion, forming an aerobic layer by multiplying aerobic bacteria in said third bacteria inhabitable portion, causing these bacteria to decompose the organic matter in said aerobic region, and moving the obligatory anaerobic bacteria flowing out of said obligatory anaerobic layer and products therefrom from said anaerobic space to said aerobic region.