We developed an automated digital gamma-imaging system for nondestructive testing of welded structures in steel pipes. The imaging system consists of a 750-m-thick CdTe (cadmium telluride) photoconductor coupled to a commercially available complementary metal-oxide semiconductor readout array having a 100 m × 100 m pixel size and a 5.4 mm × 151.0 mm active area, a collimated 75Se (selenium) gamma source having an activity of ~78.7 Ci and a physical dimension of 3.0 mm in diameter and 3.0 mm in height, and a beam limiter 1.0 mm in width and 2.6 mm in height to restrict field size. All the components were assembled with a track-typed trailer mounted around a steel pipe and moved by using a microstepping motor at a fixed speed of 5.0 mm/s. We obtained useful gamma images of some test specimens such as a duplex wire phantom and American Society for Testing and Materials batches from the imaging system and evaluated the image quality in terms of the modulation transfer function, the noise power spectrum, and the detective quantum efficiency.