A concentration detector for detecting the concentration of a particular component in a gas based upon the amount of oxygen formed by the decomposition of the particular component, comprising oxygen-forming means for forming oxygen by decomposing the above particular component, oxygen-discharging means for discharging oxygen from the gas before oxygen is formed by the oxygen-forming means, and oxygen-discharging capability-increasing means for increasing the oxygen-discharging capability of the oxygen-discharging means when the rate of increase of the oxygen concentration in the gas is greater than a predetermined rate before oxygen is formed by the oxygen-forming means.