This paper reports the intensive investigation of mass transfer near the entrance (edge) of porous media by quantification of the surrounding concentration field. We have adopted a non-invasive and real-time system based on light absorption photometry for measurement of the concentration field in a quasi-two dimensional cell. This system is, in principle, applicable to the measurement of various substances due to the generality of light absorption. This measurement system was applied to a simple model of the gravity-driven transport of a substance in a fluid near the edge of a porous medium in the presence of a reaction at the surface. The temporal variation of the complicated concentration field is appropriately captured with a spatial resolution of several tens of micrometers to millimeters. Quantitative analyses revealed that the geometry of the porous edge considerably affects the convection flow and invasion of substances into the medium.