To establish a systematic guideline for accelerator decommissioning, as a case study, beamline activation of 12 MeV-proton electrostatic accelerator was investigated employing a survey meter and γ-ray spectrometers. Beam loss points where reflected as high dose-rate area were identified, and generated nuclides and their activities were determined. Almost beamline components are made from stainless steel and 52Mn and 56Co were detected as principal induced activities. It was found that the 56Co activity significantly contribute to the dose rate value denoted on the survey meter. From the beam operation history and the monitor currents of Faraday-cups, we revealed the beam loss on a certain point significantly reflects the 52Mn activity on there. Induced activities of 52Mn and 56Co on the certain point of the beamline could be reproduced by the contact dose-rate on that point.