To obtain flat multi-hole tubes which are arranged in two or more tiers of lines of holes by extrusion processing in order to set a large amount of heat exchange easily.

A male die 10 which has a plurality of projecting parts 19, and a female die which has a die cavity in which the projecting parts are inserted, which forms a space for extrusion processing of a product between these projecting parts, are provided. The male die 10 consists of a body 11 and a core 13 which is inserted in a through hole 12 formed practically in the center of the body. The core 13 consists of a plurality of core assemblies 18a, 18b and 18c which are divided in the height direction. On a front edge of the core assemblies, a plurality of projecting parts 19 are formed integrally with a main body of the core assemblies.