To provide a water transpiration measuring instrument capable of precisely measuring an amount of moisture transpired from the skin of a subject in a short period at ant angle without being influenced by air surrounding a probe.

In order to avoid being influenced by the air surrounding the probe, after fitting the probe closely to the skin, an air circuit is closed to prevent the air surrounding the probe from entering the air circuit, air is circulated within the air circuit so as to obtain the amount of moisture corresponding to moisture to be transpired within the air circuit at any angle, and a total volume within the air circuit is reduced to quicken the responsiveness of moisture increase by circulation for reducing a time for measuring. The increase characteristic of the absolute humidity of air circulated within the closed air circuit is detected by these to measure a precise water transpiration.