The diurnal variation of water vapor over the central Tibetan Plateau during summer was investigated using various observational data and a numerical model. A systematic diurnal cycle of the precipitable water was found in the rawinsonde data. That is, the precipitable water decreased during daytime at the valley area, while it increased over the mountain range. This diurnal cycle is most significant in the pre-monsoon season but it is not clear in the middle of the monsoon season. The horizontal distribution of this diurnal cycle was examined by global objective analysis datasets and GMS-5 satellite data, and the results were found to be consistent with the topographic dependence of the diurnal cycle described above. The systematic diurnal cycle of the water vapor amount is interpreted to be caused by water vapor transport due to the thermally-induced circulation over complex terrain. This diurnal cycle of water vapor can be qualitatively simulated by a two-dimensional numerical model. The dominant horizontal scale of topography of the central Tibetan Plateau is 100 to 300 km, which is close to the most effective scale for water vapor transport as predicted by the simple two-dimensional model.