A cap for a motion guide apparatus which can reduce generation of a burr and can also increase the fixing force. The cap of the present invention is for blocking a fastening member insertion hole of a guide rail of a motion guide apparatus. The cap includes a disc-shaped top surface portion, a cylindrical side surface portion hanging down from the top surface portion, and a plurality of protrusions provided on an outer peripheral surface of the side surface portion. The plurality of protrusions is apart from each other in a circumferential direction on the outer peripheral surface of the side surface portion, and is present at an upper end portion, or from the upper end portion to an axially middle portion, of the outer peripheral surface. A thickness of the side surface portion between the protrusions is less than a thickness of the top surface portion.