A styrene-based polymer molding is obtained by molding a styrene-based polymer having mainly syndiotactic configuration and then irradiating the molding thus obtained with electron rays. This molding is excellent in heat resistance and further inexpensive. Thus the molding is useful as a material required to have high heat resistance, such as a structural material, a food container material, a food wrapping material or an electric or electronic part material, particularly as an electric or electronic part material, because it is excellent in soldering resistance.