NOVELTY - A hydroxyl-containing polyether (I1) which has a hydroxyl group and polyether chain derived from a cyclic ether compound, and an alkali metal or alkali metal compound are reacted in presence of specific plasticizer (Z1) having weight average molecular weight of lesser than that of polyether (I1), and subsequently with a halogen compound which has an unsaturated group, to introduce an unsaturated group into the terminal of polyether (I1), and to obtain a terminal unsaturated-group-containing polyether.    USE - Manufacture of terminal unsaturated-group-containing polyether used for manufacture of hydrolyzable silyl group-containing polyether used as hardening component of curable composition. Uses include but are not limited to sealing agent e.g. elastic-sealing-compound sealant for construction, sealing material for multi-layer glasses, rustproof and waterproof of glass edge portion, solar cell backside sealing agent, electrical insulation material, adhesive, coating material, film material, gasket material and casting material.    ADVANTAGE - The method enables efficient manufacture of terminal unsaturated group-containing polyether.    DETAILED DESCRIPTION - A hydroxyl-containing polyether (I1) which has a hydroxyl group and polyether chain derived from a cyclic ether compound, and an alkali metal or alkali metal compound are reacted in presence of a plasticizer (Z1), and subsequently with a halogen compound which has an unsaturated group, to introduce an unsaturated group into the terminal of polyether (I1), and to obtain a terminal unsaturated-group-containing polyether. The plasticizer (Z1) has weight average molecular weight of lesser than that of polyether (I1) and is plasticizer (Z-a) which is cyclohexane dicarboxylic acid ester, phthalic ester, epoxy-type plasticizer, polyester plasticizer, polyether-based plasticizer, polyvinyl polymer, 8-18C aliphatic hydrocarbon compound, 10-30C chlorinated paraffin. The polyether (I1) is synthesized in absence of plasticizer (Z1).