NOVELTY - Ring-breakage addition reaction of a cyclic ether compound is carried out by an initiator having 1 or more hydroxyl group per molecule in presence of a composite metal cyanide complex catalyst and specific plasticizer, to obtain a hydroxyl-containing polyether of weight average molecular weight of 5000-500000. The plasticizer has hydroxyl value of 2 or less, and weight average molecular weight of less than that of the obtained hydroxyl-containing polyether.    USE - Manufacture of hydroxyl-containing polyether used for manufacture of hydrolyzable silyl group containing polyether and manufacture of urethane prepolymer (all claimed) used as hardening component of curable composition. Uses include but are not limited to 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 hydroxyl-containing polyether with high molecular weight and low molecular weight distribution, with suppressed generation of byproduct from plasticizer. The hydroxyl-containing polyether provides urethane prepolymer with excellent workability and physical characteristics.    DETAILED DESCRIPTION - Ring-breakage addition reaction of cyclic ether compound is carried out by an initiator which has 1 or more hydroxyl group per molecule in presence of a composite metal cyanide complex catalyst and plasticizer, to obtain a hydroxyl-containing polyether of weight average molecular weight of 5000-500000. The plasticizer is a compound having hydroxyl value of 2 or less, and weight average molecular weight of less than that of the obtained hydroxyl-containing polyether, and is phthalic ester, epoxy-type plasticizer, polyester plasticizer, polyether-based plasticizer, polyvinyl polymer, 8-18C aliphatic hydrocarbon compound and/or 10-30C chlorinated paraffin. INDEPENDENT CLAIMS are included for the following:    (1) manufacture of hydrolyzable silyl group-containing polyether, which involves manufacturing the hydroxyl-containing polyether, and reacting a hydrolyzable silyl group-containing compound with the hydroxyl group of the obtained hydroxyl-containing polyether; and    (2) manufacture of urethane prepolymer, which involves manufacturing hydroxyl-containing polyether, and reacting a polyisocyanate compound with the obtained hydroxyl-containing polyether.