A butyl rubber composition is preferably used for an intermediate rubber layer 3 of a composite flexible hose formed by laminating, from the inner side, an innermost layer 2, intermediate rubber layer 3, fiber reinforcement layer 4, and an external rubber layer 5 in this order. The butyl rubber composition includes halogenated butyl rubber, styrene-isobutylene block copolymer, white filler, white reinforcing agent, brominated alkylphenol-formaldehyde resin, phenol resin, and methylol melamine, and 5 to 45 parts by weight of the styrene-isobutylene block copolymer is added to 100 parts by weight of the halogenated butyl rubber.