An oxygen carrier system includes an artificial oxygen carrier including hemoglobin vesicles encapsulating hemoglobin and an intermediate electron mediator having a standard electrode potential of 0.05 V to 0.56 V (vs. the standard hydrogen electrode) in the vesicles, and a reducing agent which is added to an external aqueous phase of the hemoglobin vesicle to reduce methemoglobin when the hemoglobin encapsulated in the hemoglobin vesicle is oxidized to the methemoglobin. The reducing agent has, as an active component, at least one compound selected from the group consisting of a thiol compound and a reducing sugar.