Mutations in the caveolin-3 gene cause autosomal dominant limb-girdle muscular dystrophy 1C (LGMD1C). However, the precise molecular pathogenesis of caveolin-3-related muscular dystrophy remains uncertain. Here, we demonstrate the effect of gene dosage on the severity of the myopathic phenotype in P104L mutant caveolin-3 (mCav3(P104L)) transgenic mice, a model of LGMD1C. We analyzed the endoplasmic reticulum (ER) stress response in the transgenic mice and found upregulated transcription of the molecular chaperone, glucose-regulated protein (GRP78). Moreover, signaling downstream of GRP78 in the myofibers was activated toward apoptosis. However, terminal transferase dUTP nick end labeling assays detected a few apoptotic nuclei in transgenic mouse skeletal muscle, probably due to the transcriptional activation of Dad1, an anti-apoptotic factor in the ER. These findings suggest that the ER stress response caused by mCav3(P104L) plays a role in the pathogenesis of LGMD1C as a toxic gain of function effect.