We reported a case of malignant lymphoma of the thyroid accompanied by interstitial pneumonitis during radiation therapy after systemic chemotherapy. Thyroid malignant lymphoma is a rare malignant tumor, which usually responds well to irradiation and systemic chemotherapy. Interstitial pneumonitis occurred during radiation after systemic chemotherapy. In this case, irradiation was simultaneously done with the chemotherapy. The radiation field included the neck and upper mediastinum. Chest X-ray showed multiple patchy shadows in both the middle and lower lung field, and it changed to contraction shadows after steroid pulse therapy. 67Ga scintigram showed multifocal increase of uptake, when chest X-ray patchy shadows were seen only in the middle and lower lung fields. 67Ga scintigraphy and CT were considered useful for evaluation of drug-induced interstitial pneumonitis.