A composition of the present invention includes: a styrene-based elastomer; a paraffin-based process oil; an olefin resin; a crosslinking agent including an organic peroxide; a crosslinking aid; an antioxidant; and a surface-treated filler obtained by surface treating particles formed from magnesium hydroxide with a higher fatty acid. The paraffin-based process oil has a kinematic viscosity at 40° C. of 300 mm2/s or greater. Furthermore, from 405 to 485 parts by mass of the paraffin-based process oil, from 9 to 13 parts by mass of the olefin resin, from 5 to 7 parts by mass of the crosslinking agent, from 13 to 15 parts by mass of the crosslinking aid, from 3 to 4 parts by mass of the antioxidant, and from 15 to 25 parts by mass of the surface-treated filler are respectively blended per 100 parts by mass of the styrene-based elastomer.