Polyarteritis nodosa (PAN) confined to the muscle is rare and hard to diagnose. Recently, the utility of magnetic resonance imaging (MRI) for detecting muscle involvement of PAN has been introduced. We herein report the case of biopsy-proven, refractory PAN confined to the lower limb muscles with enhanced MRI demonstrating discretely granular hyperintensities, which was contrary to previous reports. Our results, with those of previous reports, suggest that the MRI findings of muscles in PAN reflect the vessel size involved and disease severity.