We report a successful treatment with voriconazole (VRCZ) for chronic progressive pulmonary aspergillosis (CPPA) in a patient with dermatomyositis-associated interstitial lung disease (DM-ILD) treated with tacrolimus. A 73-year-old man with DM-ILD, treated with tacrolimus and prednisolone, complained of productive cough and his chest X-ray showed infiltration in the left upper lung field. We diagnosed CPPA and added VRCZ. Although we reduced the dose of tacrolimus for drug interaction, serum VRCZ level increased after the treatment. The patient was found to have cytochrome P450 (CYP) 2C19 *2/*2, a genetic polymorphism in poor metabolizers of VRCZ. We adjusted the doses of both drugs and treated him successfully. We recommend performing individual therapeutic drug monitoring (TDM) in CYP-mediated drug interactions and considering the effect of CYP polymorphisms.