In a method of manufacturing an intermediate shaft including: an internal shaft having an external spline; a tubular external shaft having on an inner circumference thereof an internal spline; and a resin coating provided on the external spline, a central axis line of an internal shaft manufacturing intermediate product and a central axis line of an external shaft manufacturing intermediate product are aligned with each other, and the internal shaft manufacturing intermediate product and the external shaft manufacturing intermediate product are held in such a way that the central axis lines thereof can be adjusted, in a heating adaptation step. In this state, the internal shaft manufacturing intermediate product and the external shaft manufacturing intermediate product slide relatively in an axial direction. In the heating adaptation step, a grease is interposed between the internal shaft manufacturing intermediate product and the external shaft manufacturing intermediate product.