Abstract: In a carbon black (CB)/PTFE composite porous sheet that can be used as a gas diffusion layer in an electrochemical device in applications involving electro chemical reaction such as a polymer electrolyte fuel cell, electrolysis, gas sensor and the like, wrinkle or breakage may be produced due to its flexibility. A method is provided which makes it possible to easily handle this sheet that is difficult to handle, without giving rise to wrinkle or breakage. The present invention relates to a method for laminating the composite sheet on MEA, comprising the steps of: providing a membrane electrode assembly (MEA); providing a composite sheet comprising functional powder and polytetrafluoroethylene (PTFE); providing a release film; superimposing the composite sheet on the release film and pressing them at normal temperature; superimposing the composite sheet having the release film pressed at normal temperature thereto on MEA and hot-pressing them; and separating the release film from the composite sheet.