We report the case of a 19-year-old woman who had been suffering from general fatigue and exercise intolerance since 15 years old. At 18 years old, she experienced muscle weakness and myalgia of the calves. Six months later, she was admitted to our hospital. She showed muscle weakness of the neck and proximal limbs, and myalgia of the calves was prominent. Serum levels of creatine kinase (CK) and lactic acid were elevated, as was the level of lactic acid in cerebrospinal fluid. T2-weighted and short-inversion-time inversion recovery (STIR) imaging of the lower limbs showed hyperintensity on bilateral gastrocnemius muscles, and the region revealed Gd enhancement. Based on histopathological findings from muscle and identification of a m.3271T>C point mutation, mitochondrial myopathy was diagnosed. Rest and administration of vitamins B1 and B2, coenzyme Q10, and L-carnitine improved serum CK levels; however, exercise intolerance, myalgia, and lactic acidemia remained. Sodium pyruvate was then administered, and lactic acid levels, exercise intolerance, and findings on magnetic resonance imaging improved. Sodium pyruvate could prove effective in addressing both elevated serum lactic acid levels and exercise intolerance in mitochondrial disease.