The relation of secondary injection-molding condition to the interfacial structure and shear welding strength of polyoxymethylene was investigated and following main results have been obtained. (1) The welding strength intensifies with increases of the secondary injection-molding speed and temperature. (2) With the increase in welding strength, the elongation to shear fracture becomes large also. The fracture mechanism of welding specimen changes from the interface debonding to the fracture with plastic deformation. (3) The higher the injection-molding speed and temperature, the molecule activation energy at the welding interface becomes high. So, the surface layer crystallization of the first molding is promoted and the trans-crystal layer is easy to be formed in the secondary molding side. It is thought to be advantageous to the improvement of welding strength. (4) Since the fracture starting point of welding specimen is at the welding interface or the surface layer of the first molding, the maximum welding strength is mainly determined by the strength of the surface layer.