Knowledge of radioactivity distribution is important for the decommissioning of postaccident plants. We derived methods to obtain the 137Cs contamination density of the inner surface of a pipe using a pinhole-type gamma camera, surface dose rate measurements using a Teletector, a GF10, and a lead-shielded ionization chamber. Using these methods, we obtained the 137Cs contamination density distribution of a Unit 1 standby gas treatment system outdoor pipe at the Fukushima Daiichi nuclear power station. Considering the characteristics of each method, these results were consistent. We obtained useful data for the decommissioning program.