A series of shaking table tests in 1 g gravitational field was performed for cement mixed grounds with various improvement widths to investigate the effect of cement-mixing on the static and dynamic earth pressures acting to retaining structure. The horizontal and vertical earth pressures and total horizontal force acting to a retaining structure were observed to evaluate the vertical distribution of earth pressure depending on improvement width and seismic loading. Finally, the reduction of earth pressure depending on the improvement width was evaluated to propose an optimum improvement width for cement-mixing. The main conclusions obtained from this study are as follows: (1) Cement-mixing into the ground behind a retaining structure is effective for reducing the static earth pressure acting to the retaining structure. Static earth pressure for all depth becomes 0 kPa when the improvement ratio B/H is over 0.75 (B: the improvement width of cement-mixing, H: the height of a retaining structure). (2) The amplitude of dynamic earth pressure increases with increasing input acceleration showing a peck magnitude at the middle height of a retaining structure. (3) Under current experimental condition in this paper, B/H = 0.75 can be suggested as an optimum improvement dimension irrespective of input acceleration to reduce seismic earth pressure acting to a retaining structure effectively.