We herein report the case of a 37-year-old immunocompetent man who died from Pneumocystis jirovecii pneumonia (PCP). He was initially treated for an acute exacerbation of interstitial pneumonia; however, the elevation of the patient's serum (1-3) β-D glucan (BG) level suggested the possibility of PCP and sulfamethoxazole trimethoprim was added. A postmortem pathological examination and retrospective Grocott's methenamine silver (GMS) staining of the bronchoalveolar lavage fluid (BALF), which was obtained on the day of admission, revealed PCP. The present case suggests that it is essential to perform a BG assay and GMS staining of BALF specimens when patients show diffuse ground-glass opacity on chest computed tomography, regardless of their immune status.