PURPOSE: To obtain a poly(arylene ether ketone) excellent in plasticity and heat resistance by Friedel-Crafts polymerization of a specified monomer.

CONSTITUTION: The poly(arylene ether ketone) consist of a thermoplastic crystalline polymer having repetition units of formula I. In the formula, -O- group is ether oxygen, Z group is a group except -O- group independently and at least 25% of Z group is -CO- group, Ar residue is multi cyclic aromatic residue having no adjacent ethylene carbon atoms composing a part of polymer chains, phenylene, and at least some of non-terminated Ar residues have aryl-aryl residue. The polymer is prepared by Friedel-Crafts polymerization of a monomer expressed by formulas II and III.