Different arguments have been proposed from various institutes on post installed anchor pull-out strength calculation. There are many typical formulas to calculate pull-out strength of post installed anchor. In this paper, the calculation formula for the maximum pull-out strength of the metallic expansion post installed anchor has been derived from estimation of maximum resistance by gathering statistically influential factors based on many experimental data within a scope of expansion anchor failure exclusively limited to concrete failure, and from a safety factor on calculated values given by probability theory under a hypothesis with experimental values and calculated values ratio distribution pattern taken as normal distribution because of dispersion of the experimental values.