A steam boiler, comprising a boiler body accommodating water and steam therein and a burner throwing flames toward a central through-hole of the boiler body and water pipes formed to cross the inside of the central through-hole of the boiler body, and directly heated by the flames of the burner and an internal cylinder formed to cover the outside of the boiler body so as to form a space between the internal cylinder and the outer wall surface of the boiler body where burned discharge gas flows from the lower side to the upper side and an external cylinder formed to cover the outside of the internal cylinder so as to form a space between the external cylinder and the outer wall surface of the internal cylinder where air for combustion flow such that heat exchange between the air for combustion and the discharge gas is performed.