Previously, research has been done on how to automatically generate assembly sequences utilizing 3D models of an assembled production object. Generally speaking, to assemble products, parts that have holes are prepared and the other parts are inserted into the holes. In this paper, aiming to generate a feasible sequence satisfying the insertion condition using only a STEP CAD file, we propose to use an insertion matrix indicating the insertion relation between adjacent parts. The beginning of the overall automated process extracts geometric information about the assembled product, disassembles it into individual parts, and examines interferences and insertion relationships. With this information automatically extracted from the model, the assembly sequence is optimized using a genetic algorithm. Experiments have shown that the generated sequence satisfies the insertion condition, that the male part to be inserted is assembled after preparing the female part properly.