PURPOSE: To enable the wear condition of a dresser to be easily accurately measured by providing a probe arm to move toward and away from an end of the dresser to convert the shift amount of the probe held by the arm into the electric signal to be measured.

CONSTITUTION: A rotor 12, an adjusting mechanism 15 and a probe arm 17 of a measuring head 7 are pivoted by the operation of a cylinder 8 to make a probe 18 opposed to an end of a diamond tool 5. Next, pressure in a cylinder 23 is extracted from a pipe 23a, and the probe 18 is pivoted about a pivot 19 by a force of a spring 14 to make a sintered hard alloy tip 20 touch the end of the diamond tool 5. According to the wear condition of the end of the tool 5 is lowered the probe 18 to the sintered hard alloy tip 20 side. This vertical movement of the probe 18 is converted into electric signals by a differential transformer 25. When this movement exceeds a predetermined value, the diamond tool 5 is interchanged around the grinding cycle.