Abstract: A method of determining surface pore diameter distribution of a porous material is provided. A compressed gas is charged from backside of a sample of a porous body, a surface of the sample is immersed into a liquid wetting agent, and a gas pressure is regulated from high to low, such that pore mouths of the surface of the porous body are blocked by the wetting agent in an order from small to large pore sizes, and a gas flow rate is more rapidly decreased. When the pressure is decreased to a value, the gas flow rate is reduced to zero and the pressure at this time is corresponding to the largest pore mouth size of the surface of the porous body. The pore size distribution is determined by comparing the obtained gas pressure—flow rate curve (wet curve) with a gas pressure—flow rate curve (dry curve) obtained for the sample under a dry state. The pore size and the pore distribution are calculated based on the conventional bubble pressure method.