in fig3 first of all the method comprises the step for metamorphosing a component of starch of the bean into a dextrin by heating to a temperature of over 160 degree centigrade with a heat chamber with sterilizing the beans during the heating process after a cleaning processing of the beans by washing and drying the heating means may be a heat chamber having an external heating means an electronic heating means or a rotating high pressure heat chamber in which the heating of the beans at a high temperature is effectuated during rotating and extracting of the beans at room temperature for inflating the beans as a porous product the heating temperature about the bean is different according to the heating method and nature of the bean generally the starch component of the normal bean is metamorphosing into a water soluble dextrin by heating over 160 degree centigrade and 225 degree centigrade at high pressure heat chamber arrangement due to metamorphosing a component of starch of the bean into a dextrin it changes the character of the starch and eliminates intrinsic off flavor of the beans including the grassy smell etc after the above processing the beans are ground.