PURPOSE: To facilitate positioning the rotational direction of a bobbin and cores in the direction of rotation by a construction wherein V-shaped grooves each having a required tapered part are formed in the outer peripheries of a flange of the bobbin and flat parts of the cores respectively and the end parts of required positioning members are made to face these V-shaped grooves so that they come into contact with the grooves.

CONSTITUTION: Two single unit bodies 16 each comprising a bobbin 10 and two cores 14 holding the bobbin between them are superposed and put on a mandrel 36 of a bottom tool. When sliders 40, which can be withdrawn a little, are withdrawn on the occasion, gaps are formed between the fore ends of sliders and V-shaped grooves 14d of each core and V-shaped grooves 10d of the bobbin 10. By positioning the inside diameter of the cores 14 in relation to the mandrel 36 in this state, the single unit bodies 16 are put on the mandrel 36 with ease. On the occasion when the sliders 40 once withdrawn are returned in the directions of arrows A by the returning elastic forces of spring members incorporated, subsequently, the fore ends of the sliders 40 press the V-shaped grooves 14d and 10d while tracing a tapered part of each groove.