A polyether-polysiloxane block copolymer composition is disclosed. The copolymer composition comprises (A) a polyether-polysiloxane block copolymer having in a molecule structural units as expressed by General Formula (1) below: where a terminal group thereof is an alkenyl group or the like bonded to a polyether portion. The copolymer composition further comprises (B) a liquid monool organic compound, which is (B1) a glycol ether compound having a low degree of polymerization, a terminal hydrogen substituted with a hydrocarbon group, and a secondary alcoholic hydroxyl group provided on another terminal, or (B2) a higher alcohol compound having a branched alkyl group with 12 or more carbon atoms. The copolymer composition generally does not contain a dimethyl polysiloxane at more than the mass of component (A). A manufacturing method, foam stabilizer or the like containing the copolymer composition, and a polyurethane foam are also disclosed.