A gas mixing device includes: a cylindrical portion including an upper surface which is closed; a gas outflow passage formed in a central portion of a bottom surface of the cylindrical portion, and extends downward; a plurality of gas stream guide walls disposed to be spaced apart from each other in a circumferential direction along an edge of an opening formed by the gas outflow passage in the bottom surface, and installed to be rotationally symmetrical to a center of the cylindrical portion, the gas stream guide walls protruding toward the upper surface; and a gas inlet part installed between the gas stream guide walls and an inner peripheral surface of the cylindrical portion, and into which a gas to be mixed flows.