While, in the example illustrated in FIG. 13, the surfaces 211a of the insulating layers 21a are machined flat by the cutting tool 31 of the machining apparatus 30, the flattening step may otherwise flatten the surfaces 211a of the insulating layers 21a by the grinding stone 51 of a grinding apparatus 50 as depicted in FIG. 14. When the surfaces 211a of the insulating layers 21a are machined flat by the grinding stone 51 of the grinding apparatus 50, an insulating layer 21a is sucked and held to a chuck table 52 having a holding face 52a of the grinding apparatus 50. Then, while the chuck table 52 is rotated in a state in which the grinding stone 51 of the grinding apparatus 50 contacts with the insulating layers 21a, a grinding wheel 53 is rotated to machine and flatten the surface 211a of the insulating layer 21a by the grinding stone 51. It is to be noted that a protective member such as an adhesive tape may be stuck on the insulating layer 21a on the side that is sucked and held to the chuck table 52. Consequently, the surfaces 211a of both of the insulating layers 21a provided on the front face 10a side and the rear face 10b side of the core substrate 10 can be formed flat in advance before the steps beginning with the groove forming step ST2 are carried out as depicted in FIG. 15. It is to be noted that, also when the second circuit pattern layers 22 and the third circuit pattern layers 23 are formed, the flattening step illustrated in FIG. 13 or 14 may be carried out before the groove forming step ST2 is carried out after the insulating layer formation step ST1.