The tenacity and the elongation of tire cord, especially rayon tire cord, are usually expressed as the so-called “bone dry tenacity and elongation.” By the method usually adopted for bone drying, cords are dryed in an air oven at 105-110°C for 2-3 hours for the measurement.We have studied the bone drying by infrared rays with the view of shortening the time for drying and found that the time for drying is greatly shortened (within about 30sec) and drying proceeds more rapidly and thereby the values of the tenacity are given as a larger value than in the usual bone drying.The equipment for this purpose is composed of; (I) the heatresisting steel pipe (8mmφ×500mm (1) heated at 500-600°C in surface temperature by internal electrical supply (100V-700W) (II) the metalic reflector of conduit form which has a parabolical section and of which focus is placed in the steel pipe so as to give the parallel heat rays, and (III) another reflector of the same construction which is placed at the opposite side face to face so as to collect the parallel heat ray in its focus where the tire cord passes along so as to be subjected to rapid drying.The drying speed and extent of tire cord is adjustable by the voltage of electrical supply and the surrounding temperature and also the passing speed of tire cord.The values of the tenacity are varied by the absorption of moisture which takes place during the processes from drying to measurement. This variance results from the uneven deposition of moisture (from the surface to the inner parts of tire cord) even in the same average moisture content and largely from the residual twist number (ply twist-cord twist).