A method of manufacturing a fibrous web including: preparing furnish of a thick stock component and filtrate, preparing foamed furnish by mixing a surfactant and gas into the furnish, introducing the foamed furnish to a headbox of a fiber web machine, and laying the foamed furnish on a wire or between wires of a wire section of a fiber web machine for forming the fibrous web, wherein the preparation of the foamed furnish by injecting the gas uses an injection liquid to the furnish.