The stress analysis in the semi-infinite dry sand stratum subjected to the horizontal force which relates to the seismic coefficient is performed under the critical equilibrium condition. The following facts are derived from the calculation.The mean normal stresses increase in the active state and decrease in the passive state with the increase of the seismic coefficient. Both srresses converge to one value, when the seismic coefficient equals to the friction coefficient of the sand.
　The lateral earth pressure formula in earthquakes (so-calld Mononobe-Okabe Formula) is rearranged to make clear the process through which the formula is derived.
　These results of analysis are compared with the test results obtained by the method using a box on the vibration table.