We present a case of cardiac sarcoidosis with advanced atrioventricular (AV) block in a 45-year-old female patient. Although endomyocardial biopsy and other diagnostic findings were inconclusive, gastrocnemial muscle biopsy indicated a definitive diagnosis of sarcoidosis. Whole-body gallium-67 scintigraphy was useful to confirm the suspicion of the presence of sarcoidosis. Thereafter, magnetic resonance imaging (MRI) was successful in detecting the space-occupying lesion in the skeletal muscle, leading to a positive diagnosis of sarcoidosis. It is concluded that the presence of myocardial disease, such as cardiac sarcoidosis, should be considered when advanced AV block is encountered. Even if endomyocardial biopsy and other conventional diagnostic approaches are inconclusive, the possibility of cardiac involvement in sarcoidosis cannot be denied. This case report stresses the importance of more advanced diagnostic approaches, such as whole-body gallium scanning and MRI, in clarifying the etiology of AV block. Establishing the diagnosis of cardiac sarcoidosis is important in determining whether or not to begin effective long-term treatment with corticosteroids.