Danon disease (DD) is a rare lysosomal storage disorder resulting from pathogenic variants of the lysosome-associated membrane protein type 2 (LAMP-2) gene. The disease is characterized by severe cardiomyopathy, which rapidly progresses to end-stage heart failure. This case, with DD caused by a missense variant, exhibited slow progressive cardiomyopathy and survived for an extended period despite being a male. A pathological analysis revealed that only a minority of the samples exhibited autophagic vacuoles with unique sarcolemmal features, which are typical of DD. Importantly, LAMP-2 expression was absent and the myocardial tissue contained a substantial amount of p62-positive aggregates.