The oxygen partial pressure dependence of current in CuO/ZnO heterocontact has been investigated to confirm its humidity sensing mechanism. A CuO/ZnO heterocontact was fabricated by physically pressing the sintered disks of CuO and ZnO, and its current was measured at various oxygen partial pressures under wet and dry conditions at room temperature. The current of the heterocontact was strongly dependent on oxygen partial pressure only at forward bias and wet atmosphere. Such a phenomenon originates from the humidity sensing mechanism, i.e., electrolysis of water physisorbed at the vicinity of the heterocontact. The current measurement under air with ammonia water vapor also provided one more evidence for the electrolysis function of the heterocontact.