Theoretical and experimental study of diffusive enhancement in a confined oscillating laminar flow is made. In theoretical part, based on characteristic time scale analysis, new dimensionless frequency and amplitude are proposed for eliminating discrepancies between mathematical solutions and physical understandings about enhancement of axial dispersion in a confined passageway. These two key parameters are shown to govern the phenomena without the Schmidt number for a low frequency range. Using these new variables, it may be concluded that one of underlying principles governing the phenomena is, radial molecular diffusion due to axial convective transport of frozen concentration field. Also, analytic solutions are derived for a case of rectangular duct by the Galerkin method, and the results share the same physics as those of a circular tube. In experiments, effective diffusivity representing axial dispersion in oscillating flow is measured from instantaneous axial distribution of cross-sectional averaged concentration. Concentration measurements are performed by the Schlieren interferometer with high speed CCD camera system, which cause to adapt a rectangular duct instead of a circular tube. In addtion to that, effective diffusion processes are visualized in $CO_2$-Air system. It is demonstrated that effective diffusivity depends on the axial position, and this measuring technique is proved to be more general than the older method.