The evaluation of heat transfer characteristics of a helical coil heat exchanger of the decay heat removal system, where the primary coolant flows at a low velocity through horizontal banks of the tubes in the direction of gravity, is a growing concern in the engineering safety of fast breeder reactors. The purpose of the present study is to clarify the applicability of an inviscid flow model for the prediction of the heat transfer characteristics in combined convection in the helical coil heat exchanger. A sodium experiment of combined convection is carried out using two test cylinders with a diameter of 75 mm. The local Nusselt number obtained by the experiment is found to be in good agreement with the analytical value predicted under the assumption of an inviscid flow field.