Light emitting LEDs devices comprised of LED chips that emit light at a first wavelength, and a thin film layer over the LED chip that changes the color of the emitted light. The control unit adjusts the switching-device-on period based on information indicating the shifted output-high period. The pulleys, frame, drive belt, grinding belt and electric motor are contained in a housing having a skate guidance slot such that when a skate is inserted into the slot, the bottom edge of the skate is directed to the abrasive belt at the convex grinding surface.