A high-energy X-ray beam from a synchrotron radiation source, SPring-8,was used to determine the residual stress distribution below the shot-peened surface of a carbon steel plate. By using the monochromatic X-ray beam with three energy levels of 30,60 and 72keV, the stress values were determined by the (sin)^2Ψ method with the side-inclination method (Ψ diffractometer). The stress value was estimated from the slope of the linear approximation of the relation between 2θ and (sin)^2Ψ in the range of (sin)^2Ψ=0 to 0.5. The estimated stress agreed with the distribution of the residual stress measured by removing the surface layer and repeating the X-ray measurement with Cr-Kα radiation. The nonlinearity of the (sin)^2Ψ diagram can be used to estimate the distribution of the residual stress. A new method was proposed to estimate the stress value of the distributed residual stress. The new method was a combination of the side-inclination method and the iso-inclination method (ω diffractometer) to maintain the penetration depth constant. The stress value at a given penetration depth was successfully determined by the new method.