PURPOSE: To provide a constant quality product continuously in an automatic manner before a hardness test is carried out by installing a grinding robot, having both rough and finish grinders capable of contacting workpart surfaces alternately by means of tuning motion, in the tip of an arm.

CONSTITUTION: A work W to be fed by a conveying member 1 is stopped at a specified position by the detection of a detecting member 17, and then clamped by two clamping members 15 and 16. After this clamping, a second operating member is operated, and first a roughing grinder 12 of a grinding robot 3 comes into contact with a surface 18 of the work W for rough grinding. After this rough grinding, the second operating member is operated again, and a finishing grinder 13 contacts the surface of the work W for finish grinding in place of the rough grinder 12. Thus, the surface 18 of the work W before a hardness test is automatically ground in this way.