FILTER

PROBLEM TO BE SOLVED: To provide a filter in which bacteria are prevented easily and efficiently from being deposited and bred in a filter medium layer and a sterilizing agent for sterilizing the filtration layer is prevented from flowing out to a process of the following step. SOLUTION: Filter chambers 32 and a retention chamber 30 are constituted so that filtered water flow is formed from the filter chambers 32 to the retention chamber 30 at time of filter treatment and cleaning water flow is formed from the retention tank 30 to the filter chambers 32 at the time of cleaning filter medium layers 28. Also, an injection pipe 72 of sterilizing liquid is provided in the retention tank 30. In the case of cleaning the filter medium layers 28, sterilizing liquid is injected into filtered water chambers 60 of the retention tank 30 in a state in which cleaning water flow is formed.