A design procedure for anchoring depth of the steel posts for the rockfall protection fence embedded into the plain concrete retaining wall under rock impacting, should be established by rationally considering not only experimental results but also parametric numerical results. An elasto-plastic numerical analysis method for this kind of structures has been established by the authors and by comparing with the full-scale impact loading test results. In this paper, numerical simulation on the required anchoring depth of the post of dimensions H200 × 200 × 8 × 12 cross section for 4-m high rockfall protection fence placed on the concrete retaining wall were conducted varying anchoring depth of the post: 800 mm, 700 mm, and 600 mm. It was assumed that impact force was surcharged at 2/3 height of the post from the top of the wall for the flexural action to be dominant. From this study, following results were obtained: 1) a 700 mm anchoring depth of the post was required numerically to perfectly anchor the post in the wall, and 2) the current Japanese design guidelines gives significantly conservative value from the engineering point of view based on the results obtained from this study.