To provide a method for treating drain water from an oil separation apparatus, wherein the inexpensive and compact apparatus is used while sending drain water actively.

The apparatus for treating drain water is composed of a pretreatment tank opened to the atmosphere, a water level-determining means and an airtight posttreatment tank. The posttreatment tank has: a drain water inlet, which is connected to an upstream-side pipeline through a drain check valve, at the lower part thereof; a drain water outlet, which is connected to a downstream-side pipeline through a check valve, on the upper side surface thereof; and a negative pressure generating means, which generates negative pressure according to the determination of the water level judgement means, at the uppermost part thereof. The negative pressure generating means is controlled by an air pressure control circuit according to the water level judgement.