Large-scale land reclamation projects and other projects in the Osaka offshore area, such as construction of an artificial island for a new airport, have required a new look at the consolidation characteristics of Pleistocene clay layers which have not been considered for the design. Many investigations of Osaka Pleistocene clay, which forms the uppermost part of the soil profile where the Kansai International Airport is being built, have been carried, especially an effect of soil structure on the characteristics of consolidation behavior. Osaka Pleistocene clay shows a remarkable increase in compressibility when the overburden pressure exceeds the preconsolidation pressure due to a well-developed in-situ soil structure. It is, therefore, necessary to adopt sophisticated test methods to better understand the development of deformation and porewater pressure for this type of clay retaining peculiar compressibility characteristics. In this study, a specially designed separated-type consolidometer in which the soil layer is divided into five inter-connected subspecimens, is used. Due to the high preconsolidation pressure of Osaka Pleistocene clay, the separated-type consolidometer was specially designed to allow for pressure as high as 5Mpa. Tests revealed a unique consolidation behavior especially at the loading stage exceeding preconsolidation pressure where the compressibility of soil is drastically increased and dissipation of excess porewater pressure is highly affected by yielding of the soil structure.