An extrusion die for manufacturing a tube with a plurality of small hollow portions arranged in a width direction of the tube, includes a female die for defining an outer periphery of the tube and a mandrel combined with the female die. The mandrel includes a body portion and a comb-shaped portion protruded from the body portion, the combe-shaped portion including a plurality of columnar portions desposed in a width direction of the comb-shaped portion at certain intervals. An outer periphery of a tip end portion of each of the plurality of columnar portion constitutes a bearing portion for defining an inner periphery of each of the plurality of small hollow portions of the tube. Each of the plurality of columnar portions has outside surfaces opposed in a thickness direction of the comb-shaped portion, and at least one of the outside surfaces has an inwardly dented portion behind the bearing portion, the inwardly dented portion constituting an extrusion material filling space. The at least one of the outside surfaces extending from a basal end portion of each of the plurality of columnar portions to the inwardly dented portion is formed into an inclined or curved surface for leading extrusion material to the inwardly dented portion. The at least one of the outside surfaces extending from the dented portion to the bearing portion is formed into an inwardly curved surface.