PURPOSE: To make it possible to conduct a uniform intermediate pressing and to remarkably improve the characteristics of a coil after sintering by a method wherein, when a spool material is heat-treated, the spool material is increased in diameter by using a material which increases diameter by the expansion due to sintering, transformation and the like or by using a means by which the spool is compulsively expanded when the heat treatment is conducted.

CONSTITUTION: An Ag-plated Ni-Cu alloy pipe is used for a spool and a coil is formed by spirally winding a wire material on the spool. After a stainless copper solid round bar, which is processed in conformity with the inside diameter of the bobbin, is inserted into the hollow part of the coil bobbin, a prescribed heat treatment is conducted. As the heat expansion coefficient of the stainless copper is larger then that of the bobbin, the bobbin is expanded as temperature rises, a plastic deformation takes place, its diameter becomes larger than its original diameter when it is cooled, and it is adhered to wire material. Besides, expansion of the spool itself due to heat treatment, phase transformation of zerconia, etc., subjected to plastic deformation, and expansion due to martensitic transformation are utilized.