To provide a gas detector efficiently radiating the heat of a circuit part controlling a gas detection element and capable of being miniaturized.

The casing of a gas detector 1 has a cooling air passage 23 housing a circuit board 3 controlling a gas sensor 2 other than the detection air passage 22 housing the gas sensor 2. By this constitution,, the IC 3a for a power supply generating heat on the circuit board 3 and the radiation plate 3b of the circuit board 3 can be cooled by cooling air taken in from an open air introducing port 10. Therefore, the use of a radiation plate high in radiation efficiency is eliminated to make it possible to efficiently radiate the heat of the IC 3 for the power supply and the radiation plate 3b thereof. As a result, a radiation plate low in radiation efficiency, that is, a small radiation plate can be used and the size of the gas detector 1 is miniaturized.