In the Miyagiken-oki Earthquake (1978) the mid-height beams in RC viaducts of the Tohoku Shinkansen were subjected to considerable damages. The purpose of this study was to investigate analytically and statistically the causes of occurrence of damages with different degrees in mid-height beams according to district or ground. Furthermore, the reliability theory was used to examine whether the current design method is rational against earthquake. As a result, the strong relation was observed between the degree of damages in mid-height beams and the beam yielding seismic coefficient (base shear coefficient). And the necessity of reexamination of the coefficients according to ground types in aseismic design was pointed out.