We report a 29-year-old man with limb-girdle muscular dystrophy type 2M (LGMD2M) caused by a compound heterozygous mutation of 3-kb insertion in the 3'-untranslated region and c.1073A > C (p.Q358P) mutation in exon 9 in FKTN. He had been diagnosed since childhood as having Becker muscular dystrophy based on limb-girdle muscle weakness and calf muscle hypertrophy. Loss of ambulation occurred at age 26 years and cardiomyopathy was noted one year later. Muscle biopsy at age 29 revealed dystrophic changes with loss of immunoreactivity to alpha-dystroglycan (alpha-DG), which prompted us to analyze FKTN and subsequent establishment of the diagnosis of LGMD2M. Brain MRI revealed hypoplasia of the right cerebellar hemisphere and tonsil. Dysplastic part was present in the lower medial part of the hypoplastic hemisphare, which was bordered by a deep cleft. Previously reported LGMD2M patients had mild or minimal muscle weakness in addition to dilated cardiomyopathy. In contrast, our patient had more severe skeletal muscle weakness and loss of ambulation. Treatment with 3-blockers or angiotensin II converting enzyme blockers has been reported to be efficacious for cardiomyopathy in patients with muscular dystrophy. The precise diagnosis should be established early in patients with muscular dystrophy complicated with cardiomyopathy.