To provide an oil separator further surely and effectively separating oil from water.

This oil separator 1 is provided with a vessel body 11 partitioned into four tanks of first, second, third and fourth tanks 11a, 11b, 11c and 11d for separating waste water into oil B and water (purified water) C; a waste water supply mechanism 20 having a supply pipe 21 disposed with one end opened at the bottom of the first tank 11a and supplying waste water from the opening of the supply pipe 21 into the first tank 11a; an oil recovering mechanism 25 recovering oil B staying at an upper part of the purified water C to the fourth tank 11d and discharging outside; and a discharge pipe 30 discharging the purified water C in the third tank 11c by pressure of oil B and water C in the vessel 11. The first tank 11a and second tank 11b are partitioned by a first partition member 12 with upper parts thereof communicating with each other, and the second tank 11b and third tank 11c are partitioned by a second partition member 13 with bottom parts thereof communicating with each other.