A wafer can be thinned without occurrences of dimples. A support plate 1 has a number of through holes 10. A circuit forming surface of a wafer 2 is adhered to one surface of the support plate by an adhesive member 4, and a dimple prevention member 3 having a thickness of 100 μm or more and having an adhesive layer 30 on one face is adhered to the other surface, thus the openings at both ends of the through holes 10 are blocked. The support plate is vacuum adsorbed to a support table through the dimple prevention member, and a non circuit-forming face of the wafer is ground/polished to thin the wafer. The dimple prevention member is stripped off, and a solvent is penetrated into the adhesive member through the through holes to detach the wafer from the support plate.