To provide a method for producing a carbon type porous electrode substrate for phosphate type fuel cells which has superior corrosion resistance and with which electricity generation is carried out stably at high output for a long duration.

A carbon-fiber/based material is impregnated with a solution, produced by dissolving organic polymer substance with 40% or more ash content in an organic solvent, dried, and thermally hardened to deposited the organic polymer substance on the carbon-fiber/based material. After the resultant material has been carbonized by firing at 800°C or higher temperature in an inert gas atmosphere, the material is thermally treated at 2300°C or higher temperature to form a graphite coating film on the surface layer part of the carbon-fiber/based material and then fluorination treatment is carried out. The fluorination treatment is preferably carried out by plasma reaction of a fluorine-containing gas, and it is preferable that fluorination treatment be carried out in a reaction condition of 10-minute or shorter reaction duration at 30W or lower microwave output, while leading CF4 gas to a plasma reactor.