To treat a raw food ingredient to extremely retard and prevent the oxidation of the food by heating the raw food ingredient in a device regulated so as to have a specific inner pressure.

A raw food ingredient is heated in a device regulated so as to have 0.3-5.0 kg/cm2 inner pressure to treat the objective raw food ingredient. Nuts, seeds and the like are cited as the raw food. Walnuts, pecans, peanuts, almonds, hazelnuts, cashew nuts, pistachio nuts, Brazil nuts, macadamia nuts and the like are cited as the nuts. Seeds of pine, sesame, seeds of sunflower, seeds of pumpkin, seeds of poppy, seeds of chinquapin and the like are cited as the seeds. A method of feeding air, nitrogen gas or the like within a pressure range of 0.5-1.5 kg/cm2, and the like are cited as the means for regulating the inner pressure. The treating device is preferably hermetically sealed.