A polymer composition which comprises a polymer, a phosphorus compound (a), and a phenol compound (b). The phosphorus compound (a) is, e.g., at least one member selected from compounds represented by the following formula (1). The phenol compound (b) is, e.g., at least one member selected from compounds represented by the following general formula (4), ##STR00001## wherein in the general formula (1), the R.sup.1 represents C.sub.1-20 alkyl, allyl, cycloalkyl, phenyl, or alkoxyalkyl and n is 1 or 2, provided that when two R.sup.1's are present, then the R.sup.1's may be the same or different, ##STR00002## wherein in the general formula (4), the R.sup.4's each independently represents C.sub.1-20 alkyl, allyl, cycloalkyl, phenyl, alkoxy, or alkylthiomethyl.