Abstract: A process for producing an unsaturated group-terminated high-molecular weight polyalkylene oxide by adding an alkali metal and/or an alkali metal compound capable of producing an alkali metal hydroxide on reaction with water to a hydroxyl-terminated polyalkylene oxide having a repeating unit represented by formula --R.sup.1 --O-- (wherein R.sup.1 represents a divalent alkylene group having from 2 to 8 carbon atoms) to substitute the hydrogen atom of the hydroxyl end group with an alkali metal (alkoxidation) the resulting polyalkylene oxide is reacted with a polyhalogen compound to increase the molecular weight of the polyalkylene oxide and then additional alkali metal and/or alkali metal compound is added before reacting the resulting high-molecular weight polyalkylene oxide with an unsaturated halogen compound to introduce an unsaturated group to the molecular chain terminals.