In order to study the phenomenological changes in the blast furnace accompanied with the injection of heavy oil at the tuyere, the reaction gas at the tip of the tuyere was sampled by, the use of the horizontal probe and analysed. In the case of no oil injection, CO₂ content increased with decreasing oxygen content in the inward direction gradually, and after reaching the maximum point of CO₂ content, CO and H₂ content increased. When the injection rate of heavy oil increased, these phenomena tended to proceed rapidly and to be accomplished in shorter range. When the nozzle head of the oil burner was pulled backward in the blow pipe, the decrease of oxygen occurred even in the blow pipe. The decrease of the space of the raceway accompanied with the increase of oil rate was found also by the bar-test. The gas distribution at the top of the shaft was measured by the horizontally inserted probe installed at the stockline level and the gas efficiency was calculated.