To provide a method and a device for treating radioactive organic waste liquid with enhanced decomposition ability for the radioactive organic waste liquid.

This device/method comprises a waste liquid treating vessel 2 for storing the radioactive organic waste liquid containing an organic component constituted of organic carbon (14C) emitting β-ray, and a catalyst 3 arranged inside the waste liquid treating vessel 2, and carrying platinum on zirconia, and the catalyst 3 is irradiated with the β-ray emitted from the organic carbon (14C) to decompose the radioactive organic waste liquid containing the organic component.