A high hot metal temperature is ensured even when the heat flow ratio in a lower portion of a furnace is high.
An oxygen blast furnace 1 includes a furnace body 2 into which a raw material is charged, and a tuyere 3 for injecting a blast gas and a reducing agent into the furnace body 2. The blast gas has an oxygen concentration of 50% by volume or more. A heat flow ratio β, represented by the following equation (1), in a lower portion of the furnace is 0.8 or more. The ratio (D/R) of the distance D (m) from the front end of a raceway, formed in front of the tuyere, to a furnace wall to the radius R (m) of the furnace body at the height of the tuyere satisfies the following inequation (2). β = C pig + C slag + C coke / C gas D / R > 0.13 β + 0.07 where C pig is the heat capacity (W/K) of hot metal in the lower portion of the furnace, C slag is the heat capacity (W/K) of slag in the lower portion of the furnace, C coke is the heat capacity (W/K) of coke in the lower portion of the furnace, and C gas is the heat capacity (W/K) of reducing gas in the lower portion of the furnace.