DETECTOR

An object of the present disclosure is to reduce the chance that erroneous detection occurs. A sensor 1 includes a smoke detection chamber 4, an opening 510 and a dividing portion Z1. The smoke detection chamber 4 has an inlet port 40 through which smoke flows into the smoke detection chamber 4. The opening 510 connects an external space SP2 and a space SP1 surrounding the smoke detection chamber 4. The dividing portion Z1 divides the gas flow channel 6 such that a first ratio is made higher than a second ratio. The first ratio is a ratio of an amount of smoke reaching the inlet port 40 with respect to a first inflow of smoke flowing into the gas flow channel 6 through the opening 510. The second ratio is a ratio of an amount of steam reaching the inlet port 40 with respect to a second inflow of steam flowing into the gas flow channel 6 through the opening 510.