This paper investigates the relation between swelling and decay of strength during the wet-state of the drying-wetting of diatomaceous mudstone. A dry specimen in a triaxial cell was returned to the wet state by the use of back pressure. Loss of strength was confirmed from the result of the compression test of a wet specimen, even though confining pressure was given to the specimen to prevent swelling. The degree of loss of strength obtained from these tests is approximately equal to that observed for a specimen in an inundated condition. It can be concluded that the loss of strength during the wet-state is not connected with swelling. but rather depends only on the highest degree of desiccation attained during the drying-wetting process.