A gas diffusion layer comprises a carbon sheet and a microporous layer disposed on at least one surface of the carbon sheet, and meeting the requirement “C is equal to or greater than 0”, wherein:



C, referred to as “index for simultaneous realization of a required in-plane oxygen permeation coefficient and electrical resistance”, is calculated by subtracting the product of B multiplied by 60 from A and adding 310 to the difference,
A, is the rate of oxygen permeation in an in-plane direction in a gas diffusion layer that occurs when a pressure of 0.5 MPa is applied in the through-plane direction to a surface of the gas diffusion layer to compress an arbitrarily selected region having a width of 10 mm and a depth of 3 mm in the gas diffusion layer, and
B is the “electrical resistance” that occurs when the gas diffusion layer is compressed by applying a pressure of 2 MPa in the through-plane direction.