A lubricating plug is provided which may be applied easily to the stud-type roller bearing with tracking wheel, roller, whether new or already existing, with no need of adding any modification to design of a lubrication hole. The lubricating plug is most suitable for certain replenishment of lubricant and also made slim in construction even with low cost. The lubricating plug is comprised of a radially enlarged portion of cylindrical contour adapted to make the fit-engagement with the lubrication hole in the stud, and a radially reduced portion made integral with the radially enlarged portion. An injection hole is bored throughout in both the radially enlarged portion and the radially reduced portion to accommodate detachably therein a lubricant feeder nozzle.