To provide an oil separator capable of reducing the oil concentration of waste liquid and separating and recovering all floating oil even when buoyancy of microbubbles inside treatment target liquid is small, they float in the separator and oil and water are emulsified, in the oil-water separator for surfacing and separating the oil by the oil adsorbing and floating effects of the microbubbles.

In an oil-water separation tank to which the fixed quantity of the oil-mixed waste liquid of an arbitrary flow rate is successively supplied, it is bisected in the direction of flow, a device for generating the microbubbles and a jetting nozzle are provided on the bottom part of the preceding stage, and a partition plate for making the microbubbles float without surfacing to a liquid surface even though they adsorb oil droplets by forced flow by a fixed quantity water supply means and overflow to a post stage is provided. For the post stage, a floating oil recovery chamber is provided to the upstream, a waste liquid discharge chamber is provided to the downstream communicating with the floating oil recovery chamber at the bottom part, and a waste liquid discharge port is provided to a position lower than the oil recovery port of the floating oil recovery chamber.