A semiconductor gas sensor includes a substrate having a cavity, a first insulation layer formed on the substrate, including an exposure hole formed at a position corresponding to the cavity and a peripheral portion of the cavity, a second insulation layer formed on the first insulation layer, covering the exposure hole, a heating electrode formed on the second insulation layer, being formed at a position corresponding to the cavity, a sensing electrode formed over the heating electrode, being electrically insulated from the heating electrode, a detection layer covering the sensing electrode, being capable of having a variable resistance when acting with a predetermined kind of gas, and a vent hole formed by penetrating the second insulation layer to communicate with the exposure hole, and the vent hole being capable of dissipating heat from the heating electrode in a upward direction with respect to the substrate. Thus, the exposed hole and the cavity may relieve sage of a membrane toward the cavity and may dissipate heat from the heating electrode swiftly and efficiently.