An apparatus for measuring local humidity with optical fibers is reconstructed and applied to the measurement of water-vapor concentration in a boundary layer, in which combined heat and mass transfer in free convection between vertical parallel plates occurs and the temperature distribution is not uniform. The two optical powers of 1.37um (absorbed) and 1.24um (un-absorbed) is measured simultaneously with a beam splitter and these ratio immediately computed. Then, it is able to compensate for lowering these optical powers by the beam shift due to the temperature distribution. The water-vapor boundary layer is generated between vertical parallel plates, one of which is wetted and heated. Measured distributions of water-vapor concentration at the upper end of the plates agree well with numerical ones.