We calculate the dihedral quandle cocycle invariants of twist-spins of alternating odd pretzel knots. The calculation leads us to the conclusion that there exist non-ribbon 2-knots which admit a non-trivial coloring by the dihedral quandle Rp and all of whose cocycle invariants derived from ℤp-valued 3-cocycles on Rp take value in ℤ ⊂ ℤ[ℤp] for any odd prime integer p.