Each of the composite bodies manufactured with the above-described procedure was subjected to blast treatment with alumina abrasive grains under conditions of a pressure of 0.4 MPa and a conveying speed of 1.0 m/min and cleaned, after which, electroless Ni—P and Ni—B plating was performed, and plating layers having a thickness of 8 μm (Ni—P: 6 μm+Ni-B: 2 μm) were formed on the surfaces of the composite bodies,

With respect to the composite bodies having the plating layers that were formed as described above, the maximum values of the shrinkage cavities and the hole cavities were investigated with a magnifying glass (a loupe with a magnifying power of 10 times), and the occurrence rate of shrinkage cavities and hole cavities was also investigated. The measured area was the entirety of the front surface, back surface, and side surface of the composite bodies, and the occurrence rate of shrinkage cavities and hole cavities in 100 sheets (the proportion of places where shrinkage cavities and hole cavities were observed with respect to the number of holes in 100 sheets) was investigated. In addition, the size of the shrinkage cavities and hole cavities was determined on the basis of JIS P 8145 “Paper and Paperboard—Foreign Matter Evaluation Method” and JIS P 8208 “Pulp—Miscellaneous Matter Measurement Method.” The results are shown in Table 1.