We report a case of radiation fibrosis appearing as mass-like consolidation, which was difficult to distinguish from local recurrence. A 72-year-old woman was diagnosed as having primary lung cancer (cT1N0M0 stage IA) in the right upper lobe and was treated with SBRT of 48 Gy in 4 fractions. After 12 months, mass-like consolidation appeared around the irradiated area, and after 13 months, it had increased in size. FDG-PET revealed high uptake (SUV max = 5.61) for the consolidation. CT-guided biopsy was performed, but we could not confirm the diagnosis. Considering her poor respiratory function and her age, short-interval follow-up was performed. After 15 months, the consolidation enlarged at the dorsal side, and carcinoembryonic antigen (CEA) became elevated (14.6 ng/mL). Serum KL-6 (436 U/mL) and SP-D (204 ng/mL) were also elevated. However, after 16 months, serum CEA slightly decreased. The consolidation gradually retracted on follow-up CT images. CEA, KL-6, and SP-D were also decreased by degrees. After 40 months, there is no evidence of local recurrence.