In this report, at first, the effect of the roughness of the testing surface of specimen on the Shore hardness was investigated, and then the effect of the roughness of the reverse surface was investigated. The experiment was carried out with a Shore scleroscope of D-type on many specimens, which were different in hardness or in material, under the constant operating condition of the tester throughout the experiment without being effected by the speed effect of the operating handle and the force, with which the specimen was pressed. As a result, these effects on the hardness number have been made clear, and it has been revealed that the roughness of the testing surface of specimen affects the hardness number, but on the other hand the roughness of the reverse surface does not influence the hardness number. The critical roughness curve, which shows the relation between the maximum (critical) roughness for the testing surface to free from the roughness and the hardness of specimen was given. And an experiment also conducted to find direct causes of the so-called effect of the roughness of the reverse surface of specimen is described in the present report.