A water vapor permeable membrane is provided comprising a dense layer and a support layer that are adjacent to each other, wherein the dense layer contains voids with a void length of 0.1 μm or less and the dense layer has a thickness of 0.1 μm or more and 2 μm or less while in the support layer, void (a), i.e. the void with the largest length in the 2 μm thick region measured from the boundary between the dense layer and the support layer into the support layer, has a length of 0.3 μm or more and void (b), i.e. the void with the largest length in the region ranging between 2 μm and 4 μm measured from the boundary into the support layer, has a length of 0.5 μm or more, the length of the void (b) being larger than that of the void (a). A water vapor permeable membrane having both a high water vapor permeability and a low air leakage is provided.