To enable the molding of a hollow part or a groove part of a varying shape using a common core mold when molding a concrete block with the hollow part or the groove part.

The core mold 3 is formed of a flexible mat material 10 such as rubber and numerous core pieces 13 are embedded in the mat material 10. Further, the more pieces 13 of the mat material 10 are held between a pair of retaining frames 15 and then are molded into a cylindrical form. Thus, it is possible to select between the retaining frames 15 corresponding to the shape of a hollow part to mold the mat material 10 during the molding of a concrete block by making sets of the retaining frames of a varying shape available.