A male in his sixties with locally advanced pancreatic ductal adenocarcinoma (PDAC) was administered gemcitabine plus nab-paclitaxel therapy. Computed tomography (CT) scans after five courses revealed nonspecific interstitial pneumonitis in addition to PDAC aggravation. No evidence of respiratory infection was detected, and his condition was stable and asymptomatic at diagnosis. Sputum test and interferon-gamma release assay revealed no evidence of tuberculosis. Through careful history taking, the patient was found to be taking dietary supplementation with Agaricus blazei Murill extract for approximately 1 month. Drug-induced lymphocyte stimulation tests for gemcitabine and nab-paclitaxel were negative, whereas those for Agaricus blazei Murill were positive. CT scans after withdrawal showed improved pneumonitis. These findings suggest a possibility that the dietary supplementation may lead to drug-induced interstitial lung disease (ILD). This patient indicates that pertinent diagnostic interviews are essential for the identification of drug-induced ILD.