PURPOSE: To improve the efficiency of a winding work and the high frequency characteristic of a bead core by forming grooves for a coil approximately parallel to the outer periphery of a cylindrical ferrite.

CONSTITUTION: Grooves 13 approximately parallel to the axis on the outer periphery of a cylindrical bead core 11 having a hollow part 12, and a coil is wound in the grooves 13. Grooves 24 for the coil may be formed on the upper and lower end faces of the core together with the grooves 23 formed approximately parallel to the axis on the outer periphery. Thus, the efficiency of winding work can be improved, the insertion loss of the coil for high frequency wave can be decreased, and the quality of the core can be improved.