an industrially advantageous process described below has been established wherein tetrahydrofuran and a cyclic ether copolymerizable therewith are copolymerized by the use of a very small amount of a commercially available heteropolyacid as it is untreated , the heteropolyacid of the catalyst is recovered , recycled and reused in a simple way , and a high quality polyether substantially free from catalyst contaminants is obtained . a process for producing a polyether , comprising the steps of copolymerizing tetrahydrofuran and at least one cyclic ether copolymerizable therewith by the use of an untreated heteropolyacid as a catalyst in the presence of a hydroxy compound to form a reaction liquid , contacting the resulting reaction liquid with an anion exchange resin so that the heteropolyacid contained in the reaction liquid adsorb to the anion exchange resin to remove the heteropolyacid , desorbing the removed heteropolyacid from the anion exchange resin to reuse it as the catalyst , and reusing the anion exchange resin after the desorption as it is , thus to recycle and reuse both the catalyst and the anion exchange resin .