Feasibility of the monitoring way of HTO in air to follow the tritium level in water contained in a small shallow beaker with open surface placed still in a room was discussed in this study. It is considered that the isotope exchange reaction between tritiated water in air and water in a beaker, the evaporation or condensation reaction of water at surface and the diffusion of tritium in water played the important roles in the tritium transfer phenomena through the air-water interface. Using the mass transfer coefficients obtained in the previous paper, change of tritium level in water contained in a shallow open beaker was numerically estimated and comparison with the experimental data showed good agreement. The estimated amount of tritium released from the facility with storage pools of spent fuels from a PWR power station using the method of this study showed good agreement with the observed value evaluated from the monitor of the power station