The invention provides a ss-pinene polymer which is excellent in heat resistance in spite of its having thermoplasticity and exerts excellent strength without impair in the inherent characteristics such as small specific gravity and excellent transparency. An objective ss-pinene polymer which has a weight-average molecular weight of 90,000 to 1,000,000 and a glass transition temperature of 80 DEG C or above can be obtained by polymerizing ss-pinene in the presence of a bifunctional vinyl compound.