There is provided a carbon fiber precursor polymer which is composed of a polymer comprising 50 wt % or greater of an acrylonitrile component, wherein the isotactic triad proportion of the acrylonitrile structural chain composed of the acrylonitrile component is 35 mole percent or greater based on the total triad proportion of the acrylonitrile structural chain composed of the acrylonitrile component, the carbon fiber precursor polymer allowing flame retardant treatment to be carried out slowly from a low temperature range, and having satisfactory flame retardant properties including low heat generation.