PURPOSE: To enable quality control accuracy of surface modification to be improved by allowing a specified amount of elements which gradually penetrate into an object to be treated to be measured in read time and the treatment conditions to be identified constantly.

CONSTITUTION: A surface of a metal object to be treated is modified by allowing a metal element to be penetrated from the surface within a surface modification treatment atmosphere. A test piece 2 which is of the same material and the same shape as a test piece 1 is cut off from the surface modification treatment atmosphere and is placed within a modification furnace along with the object to be treated. A change in electrical resistance difference between the test piece 1 and the test piece 2 is measured occasionally during the surface modification treatment or a data of electrical resistance difference which is obtained from it is compared with a calibration curve of the electrical resistance difference which is obtained previously and an amount of penetration elements, thus enabling the amount of penetration elements in surface modification treatment of the metal to be measured continuously by making adjustment to a proper surface modification atmosphere.