PURPOSE: To prevent a grinding wheel shaft from its over deflect ion with the grinding wheel shaft over load being detected accurately, by detecting the load on the grinding wheel shaft due to the tangential component of force of a grinding resistance and the composite force of the vertical line component of force.

CONSTITUTION: In the case of a work W being subjected to grinding by a grinding wheel 6, a grinding wheel shaft 5 is deflected by a grinding resistance, and displaced in the radial direction in bearings 2, 3. A work W is also deflected by the grinding resistance. It appears as the displacement of the grinding wheel shaft 5 opposed to 1st, 2nd displacement gages(21, 22), (21', 22') positions or the outer peripheral face of the work W, a displacement component(vertical line component) δn due to the vertical line component of force of the grinding resistance, and a displacement component(tangential component) δt due to the tangential component of force of the grinding resistance are respectively detected by the 1st displacement gages 21, 21' and the 2nd displacement ages 22, 22'. Those displacement components are input into computing elements 23, 23', the composite displacement values δ=(δn2+δt2)1/2 of both components, or the function thereof are calculated in the computing element, the composite displacement value or the function thereof is input to a comparator, the composite displacement values of both components are calculated in the computing element, and an abnormality signal is output when this value reaches an allowable value.