Abstract: A built-in heating device for a bed mattress including a base plate, a heater coupled with an upper surface of the base plate such that the heater is spaced apart from the base plate, a side member formed along an upper edge of the base plate without coming into contact with the heater, and a net-type protection cover coupled with an upper portion of the side member, wherein a plurality of support members is coupled to an upper portion of a side of the side member while being bent into the shape of as seen in a side view and separated from the upper surface of the protection cover, so as to, when the heating device is installed between bed springs in the bed mattress and pressure is applied onto the upper surface of the protection cover, prevent the protection cover from being damaged.