The present invention relates to an agent containing L-arginine, 0.2 to 20 parts by weight of L-ascorbic acid per 1 part by weight of L-arginine, and, if desired, at least one selected from the group consisting of ribonucleic acids, ribonucleotides and ribonucleosides. The agent can treat mitochondrial disease which shows a variety of symptoms caused by dysfunction of mitochondria in cells. L-arginine contained in the agent for treating mitochondrial disease of the present invention increases an NO radical level to thereby dilate the arteries. L-ascorbic acid serves to mitigate a harsh taste and acrid feeling accompanied with the intake of L-arginine and eliminate excessive NO radicals.