PURPOSE: To prepare efficiently a rod-like product which is hard or soft in whole or where the outer circumference is harder than the inner layer, by heat-treating a fiber bundle by means of a combination heating of a cylindrical heater and a hot air whose temp. are independently adjustable and by compressing and molding it into a rod.

CONSTITUTION: A hot air oven 3 is set at the exit side of a cylindrical heater 2 on the outer circumference of which an electrical heating device 1 is being provided, and a molding pipe 4 whose inner diameter is smaller than that of the cylindrical heater 2 is set successively to the hot air oven 3. A fiber bundle 5 having been supplied from the inlet of the cylindrical heater 2 is heated successively in the cylindrical heater 2 and the hot air oven 3 and thereafter pulled out while being compressed and molded in the molding pipe 4. The fiber bundles to be utilized are a thermoplastic synthetic fiber having heat adhesive property and a mixed fiber bundle of a synthetic fiber having heat adhesive property and a mixed fiber bundle of a synthetic fiber with another fiber, especially a fiber bundle containing more than 20% of a heat adhesive composite fiber consisting of a lower m.p. component and a higher m.p. component is preferable. This cylindrical heater has a role of adjusting the hardness of the outer circumference of the fiber bundle and the hot air of the hot air oven has a role of adjusting the heating temp. of the whole fiber bundle.