Illumination devices (7a) and (7b) which irradiate light having a wavelength of 1.1 μm or less are arranged on a front surface and a rear surface of a cover (8) of a dicing device (1). After a wafer is placed on a dicing stage (3), when the wafer is diced by a blade (4a) attached to a spindle (5), light is irradiated on an entire surface of an upper surface (element forming surface) of the wafer by the illumination devices (7a) and (7b). At this time, an illuminance of light on the wafer is set at 70 lux or more and 2000 lux or less. By this means, during a dicing operation, an area to be a light-shielded area by the spindle (5) or the like is not present on the wafer.