Abstract: A water vapor distribution measurement apparatus comprises: a light source that emits near-infrared light; a near-infrared light measurement device that is located across a measurement space from the light source and that measures the near-infrared light; an optical system that expands and applies the near-infrared light emitted from the light source in the measurement space in which a cross-section of the measurement space perpendicular to a direction connecting the light source to the near-infrared light measurement device has an area; and a distribution deriving means for deriving a water vapor distribution in the cross-section of the measurement space on the basis of a measurement result obtained by the near-infrared light measurement device. Water vapor in a measurement region having a prescribed size can be measured by this water vapor distribution measurement apparatus.