Background: The standard treatment for locally advanced non-small cell lung cancer (NSCLC) is chemoradiotherapy (CRT) followed by treatment of durvalumab, one of immune checkpoint inhibitors (ICI). Interstitial lung disease (ILD) is a clinically life-threatening toxicity of CRT or durvalumab. The patient-characteristics or dose-volume histogram parameters of radiotherapy have been reported to be the risk factors for radiation-induced pneumonitis. However, the risk factors for ILD during durvalumab therapy has not been established. Interstitial lung abnormalities (ILA) are generated by aging or smoking, and manifest as minor interstitial shadow on lung computed tomography (CT). We previously reported that ILA were risk factors for ICI-induced ILD in patients with advanced NSCLC, as well as non-lung cancer. Therefore, we investigated whether ILA could be risk factors for ILD during the durvalumab therapy. Methods: We retrospectively enrolled NSCLC patients who received durvalumab after CRT at 10 institutions from July 2018 to June 2021. Patient-information, patient-characteristics, dose-volume histogram parameters, chest CT findings, and laboratory data, were obtained. CT findings were examined using CT obtained after CRT and prior to durvalumab therapy. Results: A total of 153 patients were enrolled, and the prevalence of ILA was 37.8% (56 patients) before durvalumab treatment. Among the enrolled patients, 94 (63.5%) developed ILD during durvalumab therapy. The proportion of patients with grade 1, grade 2, or grade 3 ILD was observed to be 29.7% (44 patients), 25.7% (38 patients), and 8% (12 patients), respectively. Univariate logistic regression analysis revealed that higher age, higher dose volume histogram parameters (V5, V20, mean lung dose), and the presence of ILA were significant risk factors for grade 2 or more ILD. Multivariate logistic regression analysis showed that ILA, especially ground grass attenuation in ILA, was an independent risk factor for grade 2 or more ILD (odds ratio: 7.02, 95% CI: 2.95-16.69, p < 0.0001). Conclusions: Pre-existing ILA are risk factors for ILD during durvalumab treatment after CRT. This observation is consistent with previously reported findings in patients with advanced lung cancer and non-lung cancer. Therefore, we should pay more attention to the development of grade 2 or more ILD during durvalumab treatment in patients with ILA.