A mixture including a high specific gravity masterbatch pellets group (A) with specific gravity within a range from 1.8 to 3.5, and a low specific gravity masterbatch pellets group (B) with specific gravity within a range from 0.5 to 1.6. The high specific gravity masterbatch pellets group (A) comprises m different columnar masterbatch pellets, each with a different specific gravity, and the low specific gravity masterbatch pellets group (B) comprises n different columnar masterbatch pellets, each with a different specific gravity. The high specific gravity masterbatch pellets group (A) and the low specific gravity masterbatch pellets group (B) satisfy a formula (1) shown below:.times..times..ltoreq..times..times..times..times..ltoreq..times..t- imes..times. ##EQU00001##