PURPOSE: To surely suppress the lowering of the activity of a lipase agent after the use for a long period with a simple operation and to reduce the cost by treating a substrate with acid clay, thereby efficiently modifying an oil and fat with the lipase agent.

CONSTITUTION: A substrate such as oil and fat or a fatty acid ester is treated with an acid clay having an acidity of 1.2-10.0, preferably 3.0-8.0 before modifying the oil and fat with a lipase agent. The treatment of the substrate with the acid clay is preferably carried out by adding 0.4-4 pts.wt. of acid clay to 100 pts.wt. of the substrate and keeping the mixture at 105-115°C for 15-60min under reduced pressure.