To provide a diffusion furnace heating a semiconductor wafer on a boat for uniform characteristic.

The diffusion furnace comprises a furnace tube 12, a heater 3 on external wall of the furnace tube 12, a lid member 4 provided to freely open and shut a furnace opening 12b of the furnace tube 12; and introduces the boat 5 mounting a number of semiconductor wafers (w) to the furnace tube 12 to heat the semiconductor wafers (w) with the heater 3. In the furnace, a second furnace opening 12c is formed opposite to the furnace opening 12b of the furnace tube 12, a second lid member 13 is disposed with the second furnace opening 12c capable of universally opening and shutting it, the boat 5 mounting a number of the semiconductor wafers (w) is fed from the furnace opening 12b to the furnace tube 12 to heat the semiconductor wafers, and the boat 5 is introduced outside after the heating through the second furnace opening 12c.