To provide an acrylonitrile-based polymer suitable for a raw material for a carbon material such as carbon fiber, and having stable thermal oxidation reactivity.

This acrylonitrile-based polymer has x within the range of 0.03-0.6, and satisfies the inequality (1): 0.1887ln(x)+0.6776<y<0.1887 ln(x)+0.8776...(1) [wherein x is an absorbance ratio Da/(Da+Db) (wherein, Da is the absorbance of the absorption caused by the carbon-oxygen double bond of a carboxy group in an infrared absorption spectrum measured by a KBr tablet method; and Db is the absorbance of the absorption caused by the carbon-nitrogen triple bond of a cyano group); and y is the inverse number 1/tp of an exothermic peak appearance time tp (min) in an isothermal exothermic curve measured by a heat flux-type differential scanning calorimeter in an air flow of 100 ml/min at 230°C].