The characteristics of oscillatory boundary layer flow have been treated with keen interest during the last decade from the various aspects. Among the previous results the theoretical treatment by K. Kajiura must be the most important and fruitful one to advance in our knowledge on the present phenomena. On the other hand the senior author has had a real interest in the behaviour of sediment particles due to oscillatory fluid motion, and has conducted his systematic investigations on the sediment movement in nearshore area. The aim of this paper is to introduce some results of the recent investigations conducted at the Coastal Engineering Laboratory, University of Tokyo, with the intention of investigating the applicability of Kajiura's theory to the oscillatory flow in the vicinity of bottom with sand ripples. In order to accomplish the above purpose, the authors applied the hydrogen bubble technique to measure accurately as much as possible the time history of velocity distribution especially in the very thin layer above the bottom boundary associated with the progressive waves. The above measuring technique was verified to be enormously powerful even in the case of unsteady flow. The bottom conditions tested in the present investigation were of 1) hydrodynamically smooth bottom, and 2) rough bottom with artificial ripples. The pictures of hydrogen bubble lines released successively from a fine platinum wire were taken by using a 16 mm cinecamera specially equiped, and were analyzed flame by flame to take the digitalized data of horizontal velocity of fluid. The data were analyzed in Fourier series and the first mode of the series was only taken to compare with the numerical curves obtained through the Kajiura's theory. The horizontal velocity amplitude and its phase, the eddy viscosity and the shear stress at each elevation above bottom were calculated by using not only the experimental data of present investigation, but also the laboratory data obtained by I. G. Jonsson. The agreement between the analyzed data and the theoretical curves is basically consistent; this fact verifies the usefulness of the Kajiura's theory. Even though, there are some questions in the theoretical treatment which are arising out of the present investigations. The accumulation of more accurate data is basically requested to clarify these problems. Lastly the comparative study on eddy viscosity averaged over wave cycles in the neighbourhood of sand ripples was established by using the laboratory data of suspended sediment concentration.