The objective of the present invention is to provide a method which is for producing a gas diffusion electrode substrate having a high conductivity and a chemical resistance, and by which an increase in production cost can be suppressed. The present invention is a method for producing a gas diffusion electrode substrate in which a microporous layer is formed in a conductive porous body formed by bonding carbon fibers to each other by means of a cured product of a binder resin, the method having, in the following order: a binder resin impregnation step in which a carbon fiber structure is impregnated with a binder resin composition to obtain a pre-impregnated body; a coating step in which the surface of the pre-impregnated body is coated with a microporous layer coating solution; and a heat treatment step in which the pre-impregnated body that has been subjected to the coating step is heat-treated at a temperature of at least 200° C., wherein the binder resin composition is a liquid composition including a binder resin and a carbon powder, the binder resin being a thermosetting resin, and the method does not have a step for heat-treating the pre-impregnated body at a temperature of at least 200° C., between the binder resin impregnation step and the heat treatment step.