To interpret dynamic and static Young's moduli of soft sedimentary rocks widely distributed in Japan, the sandy shale model of the binary sand/shale mixture model are applied to the measured Young's moduli obtained from velocity logging data for the dynamic modulus and from laboratory mechanical test data for the static modulus. The Young's modulus for the rock is calculated by the Hashin-Strikman lower bound, Hertz-Mindlin contact model and Gassmann's equation. For modelling static Young's modulus, in the Hertz-Mindlin contact model, the shear modulus is calculated by incorporating the mixture of frictional and frictionless grain contacts into the model. The calculated dynamic and static Young's moduli are well consistent with the measured data for three different soft sedimentary rocks. This result demonstrates that the sandy shale model can be used to predict the static moduli required in civil engineering applications from dynamic ones obtained from seismic velocities.