Cold atmospheric pressure plasma has been intensively studied due to growing interest in biomedical applications. For example, it has been revealed that the plasma-irradiated cell culture medium (PAM: Plasma Activated Medium) shows antitumor effect as well as the direct plasma treatment. Long-lived reactive oxygen and nitrogen species (RONS), such as hydrogen peroxide (H2O2), nitrite (NO2
-), and nitrate (NO3
-), are considered as a main product in PAM. Therefore, investigation of the production characteristics of RONS in PAM with different experimental parameters is important. Here, we focused on humidification of feed gas (argon or helium) and effect of gap between the nozzle of the plasma jet and liquid surface. As a result, H2O2 concentration was increased by humidification of both feed gases. However, the concentration of NO2
- and NO3
- was decreased by the humidification of helium. In addition, the remarkable effect of the humidification on H2O2 production was observed when the plasma jet was in contact with the liquid surface.