Provided is a sealant material composition that makes it possible to ensure good sealing properties and to suppress sealant flow due to traveling. As a sealant material composition forming a sealant layer disposed on an inner surface of a pneumatic tire, a sealant material composition comprising from 1 part by mass to 40 parts by mass of an organic peroxide and from 0.1 parts by mass to 40 parts by mass of a crosslinking agent, being blended, per 100 parts by mass of a rubber component containing a chlorinated butyl rubber, is used.