A machined shape of a bore is based on the deformation amount of a data-acquisition bore after fastening of cylinder head. A cross section of an approximate shape is set to be an approximately-true circle shape, and a diameter of the approximately-true circle shape is changed along a central axial direction in accordance with the deformation amount of the data-acquisition bore, to determine the approximate shape. The approximate shape has a cross section having the approximately-true circle shape and has a simple shape which is symmetrical about the central axis. Since the machined shape of the bore is the shape obtained by reversing a phase of a recess and a projection of the approximate shape about a predetermined cylindrical shape, the machined shape has a cross section having the approximately-true circle shape and has a simple shape which is symmetrical about the central axis.