PROBLEM TO BE SOLVED: To provide a method for freezing biological materials in which the effect of a frost damage protecting agent is shown more after thawing.SOLUTION: A biological sample 1 is dehydrated to the inside by microwave reduced pressure dehydration by use of a microwave reduced pressure dehydration apparatus (step S1). After the dehydration treatment, the biological sample 1 taken out of the vacuum chamber of the microwave reduced pressure dehydration apparatus is soaked in a frost damage protection liquid (step S2). Finally, the sample is subjected to a freezing treatment by a freezer (step S3).