A surface (S) side is irradiated with an SF6 gas plasma to etch a semiconductor wafer (1) which has been peeled off in street portions, and divide the semiconductor wafer into a plurality of individual semiconductor chips. A removing agent (16) is subsequently supplied from the surface (S) side. At that time, it is preferable that the semiconductor wafer 1 divided into the plurality of chips is rotated at high speed. Accordingly, a mask material layer (3b) remaining on the surface (S) is removed by the removing agent (16). Moreover, the removing agent (16) is preferably an organic solvent, andmore preferably, methyl ethyl ketone, ethanol, ethyl acetate, or a combination of these.