A 47-year-old woman with respiratory failure was transferred to our hospital. She had been treated with high-dose prednisolone for suspected adult-onset Still’s disease, but had not been given prophylactic sulfamethoxazole/trimethoprim. Therapy for Pneumocystis pneumonia (PCP) was initiated immediately on arrival at our hospital. Pneumocystis jirovecii was detected in her bronchoalveolar lavage fluid (BALF) by polymerase chain reaction. Although she recovered from respiratory failure, her systemic exanthem worsened. She was then diagnosed with clinically amyopathic dermatomyositis (CADM) rather than adult-onset Still’s disease, because the skin findings were characteristic of the former and anti-MDA5 antibodies were detected. Computed tomography revealed that the CADM was accompanied by interstitial pneumonia. Although there was clinical improvement in the PCP and a transient decrease in serum β-D-glucan levels with sufficient administration of sulfamethoxazole/trimethoprim, β-D-glucan levels increased again. After a possible recurrence of PCP was ruled out because Pneumocystis jirovecii was not detected in the BALF, tacrolimus was administered in addition to prednisolone, and the disease progression of CADM with interstitial pneumonia was successfully controlled.