I present the experimental data obtained using various types of concentric double pipes, in particular, the loss of total head through a turning flow region from the outlet of an annulus between the inner and the outer pipes to the inlet of the inner pipe. They are given in terms of a nondimensional clearance length L/di (L is the distance between the annulus outlet and bottom of the outer pipe and di is the inner diameter of the inner pipe) for various values of area ratio and Reynolds number. It should be noted that all curves of the cofficient of head loss plotted against L/di have the maximum value at about L/di=0.625 and the minimum value at about L/di=0.250 in the Re range from 4.4×103∼3.1×104. It was visually clarified that the increase of head loss against L/di=0.625 was due to the vortex.