To provide a method for diagnosing corrosion in a power transmission line by using gamma-ray CT(computerized tomography), capable of providing high resolution by a simple structure and moreover capable of diagnosing the state of corrosion of the interior.

A gamma-ray source 1 for emitting gamma rays and a detector 2 for detecting the gamma rays are housed in a container 4 surrounded by a gamma-ray shielding wall 3. The container 4 is mounted on the power transmission line 5 so as to pass the transmission line therethrough. While the container 4 is rotated around the transmission line 5, the transmission line 5 is tomographically scanned by detecting the gamma rays transmitted by the transmission line 5.