A spline connection which comprises a spline boss having inward spline teeth; and a spline shaft engaged slidably with the spline boss and having outward spline teeth is disclosed. A header passage for a lubricant is formed along a core of the spline shaft and oil passages are further formed in the spline shaft in such a manner as to pierce therethrough radially from the header passage. At least one of the outward spline teeth of the spline shaft is provided at the top thereof with a groove crossing transversely the outer spline tooth so as to have the open end of the oil passage disposed in the groove.
