To improve a blocking resistance and a shelf stability by using amorphous polyester having a specified acid value as a principal component of a shell resin.

Amorphous polyester having ≤9.5KOHmg/g acid value is used as the principal component of a shell resin. This polyester is obtd. by condensation-polymerizing one or more kinds of dihydric alcohols, one or more kinds of dicarboxylic acids and one kind of polyhydric alcohol having ≤3 OH groups and/or one kind of polycarboxylic acid having ≤3 COOH groups as constituent monomers. When the polycarboxylic acid having ≥3 COOH groups is used, the amt. is regulated to 0.01-2wt.%, preferably 0.05-1.5wt.%, especially preferably 0.1-1.0wt.% of the total amt. of the constituent monomers of the amorphous polyester. The polyester is contained in the shell resin by 50-100wt.% of the total amt. of the resin.