A structure for closing a track rail mounting hole is disclosed, which prevents a closing cap from being press-fitted with an inclined state into the rail mounting hole, thereby being capable of positively preventing formation of a gap between the rail mounting hole and the closing cap. The closing cap includes a fitting portion having an outer diameter larger than an inner diameter of the rail mounting hole, to be press-fitted into the rail mounting hole; and a leading end portion to be inserted into the rail mounting hole before press-fitting the fitting portion into the rail mounting hole, for provisionally positioning the closing cap with respect to the rail mounting hole.