A core contains a thermally expandable microcapsule. A layer composed of a material for hollow molded article production purposes is formed on the surface of the core. The core is expanded and collapsed by heating the core to a temperature that is equal to or higher than a temperature at which the expansion of the thermally expandable microcapsule contained in the core starts. An expansion/collapse product derived from the core is discharged from the inside of a hollow molded article that is formed from the material for hollow molded article production purposes toward the outside of the hollow molded article.
