A 63-year-old woman had been employed for more than 30 years as a worker with mats made of marsh plants known as rushes. She had been exposed to clay dye, known as “Sendo dust,” during those years, and in 2001 was diagnosed with rush dust (“Igusa Sendo”) pneumoconiosis. A new nodule was later detected in the left upper lobe, and she was admitted to our hospital in 2006. A chest CT showed a pulmonary nodule with spiculation in the left upper lobe, and small nodules were also seen in both lung fields; these nodules were considered to be results of the pneumoconiosis. Although an FDG-PET scan showed only mild uptake in the nodule in the left upper lobe, the maximal SUV was slightly higher in the latter phase, which could suggest lung cancer. Therefore video-assisted thoracoscopic surgery (VATS) was performed. The pathological diagnosis of the main nodule was adenocarcinoma surrounding a pneumoconiotic nodule formed by rushes. Because the tumor overlapped the small nodules consisting of the pneumoconiosis, a differential diagnosis was difficult from the radiological findings before VATS.