A detector comprising: a first sensor configured to measure a concentration (Cs) of smoke in air; a second sensor configured to measure a concentration (Cc) of carbon monoxide in air; a processing device configured to determine whether a predetermined condition is satisfied with regard to the concentration (Cs) of smoke measured by the first sensor and the concentration (Cc) of carbon monoxide measured by the second sensor; and a notifying device configured to output a notification signal when the predetermined condition is satisfied, wherein: the processing device is configured to select either a first condition or a second condition as the predetermined condition based on a result of determination of whether a switching condition for the concentration (Cs) of smoke is satisfied, the first condition being for either the concentration (Cs) of smoke or the concentration (Cc) of carbon monoxide, and the second condition being for both the concentration (Cs) of smoke and the concentration (Cc) of carbon monoxide; the switching condition is that an amount of change (ΔCs) in the concentration (Cs) of smoke within a predetermined reference time period is equal to or larger than a first determination value; and the processing device is configured to determine whether the switching condition is satisfied. a first sensor configured to measure a concentration (Cs) of smoke in air; a second sensor configured to measure a concentration (Cc) of carbon monoxide in air; a processing device configured to determine whether a predetermined condition is satisfied with regard to the concentration (Cs) of smoke measured by the first sensor and the concentration (Cc) of carbon monoxide measured by the second sensor; and a notifying device configured to output a notification signal when the predetermined condition is satisfied, wherein: the processing device is configured to select either a first condition or a second condition as the predetermined condition based on a result of determination of whether a switching condition for the concentration (Cs) of smoke is satisfied, the first condition being for either the concentration (Cs) of smoke or the concentration (Cc) of carbon monoxide, and the second condition being for both the concentration (Cs) of smoke and the concentration (Cc) of carbon monoxide; the switching condition is that an amount of change (ΔCs) in the concentration (Cs) of smoke within a predetermined reference time period is equal to or larger than a first determination value; and the processing device is configured to determine whether the switching condition is satisfied. the processing device is configured to select either a first condition or a second condition as the predetermined condition based on a result of determination of whether a switching condition for the concentration (Cs) of smoke is satisfied, the first condition being for either the concentration (Cs) of smoke or the concentration (Cc) of carbon monoxide, and the second condition being for both the concentration (Cs) of smoke and the concentration (Cc) of carbon monoxide; the switching condition is that an amount of change (ΔCs) in the concentration (Cs) of smoke within a predetermined reference time period is equal to or larger than a first determination value; and the processing device is configured to determine whether the switching condition is satisfied.