A thermoplastic polymer containing glutaric anhydride units of the following general formula (1) and having an absorbance at a wavelength of 280 nm of at most 0.5 (the absorbance is a value of the polymer film having a thickness of 100 .mu.m, measured with a UV-visible spectrophotometer) and a glass transition temperature of not lower than 130.degree. C. has high-level colorless transparency and good heat resistance, moldability and dwell stability. ##STR00001## wherein R.sup.1 and R.sup.2 are the same or different and each represents a hydrogen atom or an alkyl group having from 1 to 5 carbon atoms.