To provide an apparatus for treating oil- and fat-containing wastewater for the purpose of degrading and treating oils and fats within a grease trap, which has the advantages from the viewpoint of a high versatility, and also from the viewpoints of cost and maintenance, and a method of treating oil- and fat-containing wastewater, which efficiently degrades and treats oils and fats within the grease trap. An apparatus for treating oil- and fat-containing wastewater 100 is an apparatus for treating oil- and fat-containing wastewater which degrades oils and fats of oil- and fat-containing wastewater within a grease trap by the action of immobilized lipase E and a microbe utilizing an organic matter, which comprises a container body 60 having an intake port 61 for incorporating oil- and fat-containing wastewater rich in oils and fats stored in the upper layer within the grease trap which has been absorbed and taken out and an discharge port 62 formed in a way that the wastewater is capable of being removed while part of the wastewater degraded and treated has been remained, an enzyme holder 70 provided and installed within the container body 60 containing the immobilized lipase E while preventing the immobilized lipase E from being flown and lost, in which the above-described oil- and fat-containing wastewater can pass through and agitating means 75 for agitating the above-described oil- and fat-containing wastewater within the above-described container body 60 and it is provided and installed exterior of the grease trap.