A thermosetting composition comprising the following component (A) and component (B), and having a viscosity of 0.1 Pa·s or higher and 100 Pa·s or lower at a shear rate of 10 s−1 at 25° C. measured in accordance with JIS K7117-2. (A) polybutadiene having a structural unit represented by the following formula (1A) and a structural unit represented by the following formula (1B), wherein the terminal group comprises a methacryloyl group or an acryloyl group (B) a thermal polymerization initiator