A new electric wire for a railway train with three coating layers has been developed to reduce the diameter and the weight instead of conventional one with two coating layers. In this threelayer coating, it is necessary to extrude the wire coating thickness with higher accuracy than the conventional two-layer coating. For that reason, the flow channel structure of the cross-head of extruder was optimized to realize precise multi-layers coating. In order to make the thickness of the wire coating uniform, we focused on the pressure drop in the crosshead channel and derived a design equation for the channel structure considering the taper angle based on fluid dynamics. Furthermore, a high viscosity silicone oil containing plastic particles was flowed in an acrylic transparent crosshead, and the flow behavior was observed. Flow velocities at every circumferential position were equal, the validity of the flow channel design equation was verified. Using the crosshead designed by this equation, the three layers coated wire was manufactured, and the dimensional target of the thinnest electrical insulation layers, 120± 10μm, was achieved. The outer diameter and weight of the three layers coated wire can be reduced by 26% and 16% compared to the conventional two layers coated wire.