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