PROBLEM TO BE SOLVED: To provide a method for removing cyanides efficiently to a predetermined value or lower from flaxseeds while preventing oil from oxidation.SOLUTION: The method includes the following steps of: housing a predetermined amount of flaxseeds in a can body (S1) heating the sealed can body (S2) steaming and heating the can body while maintaining the can body temperature at 90-115°C for a predetermined period (S3) vacuuming the can body after steaming and heating and vacuuming and heating the can body under vacuum with a set highest temperature at 80°C for a predetermined period.