a pair of disk-shaped grindstones that face each other across a prescribed gap to be rotatable in opposite directions about their center axes, wherein a grinding surface of one of the grindstones is formed to slope in a manner of an umbrella from the center of the disk or from a point a prescribed distance away from the center of the disk, and a center axis of the one grindstone is inclined so that the sloped grinding surface of the one grindstone lies parallel to a wholly flat grinding surface of the other grindstone, thereby forming: (i) a planar grinding region A where both of the grinding surfaces of the pair of grindstones lie parallel with a constant intervening gap therebetween, and (ii) a wedge-like opening region B where a gap between the grinding surfaces in the wedge-like opening region B becomes narrow toward the planar grinding surface A, and

a feeder to feed wafers from the wedge-like opening region B toward the planar grinding region A in order to grind both surfaces of the wafers at the planar grinding region A where they come into surface contact with both of the oppositely rotating grinding surfaces, wherein the feeder has a series of square openings in the longitudinal direction in which the wafers are mounted.