To investigate the relation between the roughness of hull surface and increase of skin frictional resistance, following procedures. were taken; i) preparation of a portable roughness recorder, ii) roughness measurements of hull surface of newly built ships and of a ship after 1 & 5 years service, iii) resistance test of steel plates similarly treated as actual ship hull, iv) the same plates were submerged in sea water for 10 to 49 months, v) resistance test of the submerged plates after cleaned and repainted, as ships in docking Main results of the above research are summarized as follows. a) Roughness of newly built hull are expressed by H _??_ (3060) λ^0.7, where H : apparent amplitude in micron. λ : apparent wave length in centimeter. b) Roughness configuration of the ship in service and of the submerged test plates are coincident in tendency. From this, the results of our resistance tests can be applied to estimate the augmented resistance of actual ships. c) Approximate relation between Δ C_fs and Δ C_fm , can be expressed by Δ C_fs _??_Δ C_fm · ( C_fs / C_fm ) ²· where Δ C_f : augmentation of C _f due to roughness, C _f : frictional resistance coefficient of smooth surface, “s”, “m” : suffix representing “ship” and “model”.