An optical device wafer has a device area where a plurality of optical devices are formed on the front side of a sapphire substrate, and a peripheral marginal area surrounding the device area. The device area projects from the peripheral marginal area. A break start point is formed on the front side of the sapphire substrate by applying a laser beam along the boundary between the device area and the peripheral marginal area. A protective member is attached to the front side of the optical device wafer. The optical device wafer is held on a chuck table of a grinding apparatus so that the protective member comes into contact with a holding surface of the chuck table. The back side of the sapphire substrate is ground to reduce the thickness thereof to a predetermined thickness.