On using the far-infrared radiation system, whether the irradiation effect is thermal or non-thermal is controversial. We irradiated amyloid peptides that are causal factors for amyloidosis by using a submillimeter wave from 420 GHz gyrotron. Fluorescence reagent assay, optical and electron microscopies, and synchrotron-radiation infrared microscopy showed that the irradiation increased the fibrous conformation of peptides at room temperature for 30 min. The temperature increase on the sample was only below 5 K, and a simple heating up to 318 K hardly induced the fibril formation. Therefore, the amyloid aggregation was driven by the far-infrared radiation with little thermal effect.