[en] ⁶³Ni(β^- decay, half-life; 100.1 y) is abundantly produced in hardware composed of SUS and copper at high energy accelerators. The measurement method of its activity by a liquid scintillation counter was developed. Its rough activity could be estimated on the basis of thermal and fast neutron fluxes measured by an activation method using gold and copper activation monitors. After a few tens of years from beam-off, no radiation level on these accelerator hardware does not necessarily mean that all activity had decayed out, there is a possibility that an appreciable amount of ⁶³Ni still presents in them. (author)