In a water treatment system, micro-nano bubbles are used in a production device and a detoxification device as one example of the upstream treatment devices which perform specified treatment with use of water. The micro-nano bubbles are used again in the waste water treatment by a waste water pretreatment device and a waste water treatment device in the subsequent step. Since the micro-nano bubbles are reused, the efficiency of the micro-nano bubbles can be enhanced. Moreover, according to the waste water pretreatment device, treatment water is pretreated with the microorganisms activated by micro-nano bubbles and propagating in a polyvinylidene chloride filler material and then it is introduced into the waste water treatment device in the subsequent step. Thereby it is possible to reduce the waste water treatment load in the waste water treatment device in the subsequent step.