A method of manufacturing a multi-layer resin material by employing a plurality of kinds of thermoplastic resin. Resins for inner and outer layers are supplied into a feed block of a feed block type T-die coextrusion apparatus where they are laid up another, and are then sent to the T-die side. In this case, a resin which is practically similar to the resin for the outer layer is supplied as an auxiliary resin to both lateral edges of the multi-layer resins within the feed block. Thus, the sheet extruded from the T-die slit has lateral edges which are made of a single resin.