A processing method of a device wafer includes a mask coating step of coating a front surface of the device wafer with a water-soluble resin, a mask forming step of applying a laser beam along each division line, forming a groove, and removing a protective mask and a functional layer to expose a substrate, a plasma etching step of forming a division groove that divides the substrate along the groove by supplying a gas in a plasma condition, an expanding step of expanding a protective tape in a plane direction to expand a width of the division groove, an adhesive film dividing step of applying a laser beam along the division groove to divide the adhesive film that has been exposed due to the formation of the division groove, and a cleaning step of cleaning and removing the water-soluble resin.