Artificial red blood cell having ability to inhibit transformation of hemoglobin into methemoglobin

The present invention inhibits, in an artificial red blood cell containing purified and enriched hemoglobin that substantially has no enzymatic activity to reduce methemoglobin, transformation of the hemoglobin into methemoglobin. Provided is an artificial red blood cell which comprises: an aqueous solution that contains NADH and/or NADPH and hemoglobin; and a capsule that includes the aqueous solution, wherein the aqueous solution and the capsule substantially have no enzymatic activity to reduce methemoglobin.