A resin composition comprises a resin, a vulcanization-activating agent, and a stabilizer. The vulcanization-activating agent may be a compound having a plurality of polymerizable unsaturated bonds (e.g., a compound having a plurality of (meth)acryloyl groups). The stabilizer may be an antioxidant, or a light stabilizer (particularly, a stabilizer is capable of capturing a radical). Use of the resin composition as a resin material ensures to directly join between a resin and a rubber certainly and firmly. The proportion of the vulcanization-activating agent may be about 0.1 to 10 parts by weight relative to 100 parts by weight of the resin. The proportion of the stabilizer may be about 0.01 to 5 parts by weight relative to 100 parts by weight of the resin.The present invention provides a resin composition excellent in adhesiveness to a rubber.