A contaminant detector includes a detection object that is inclined relative to a horizontal direction in a drainage unit for accumulation of discharged water and on which a contaminant is to be deposited and a detection unit configured to emit energy toward the detection object to detect a contamination state of the detection object. The detection unit includes a transmission and reception module configured to emit and receive energy, a calculation module configured to calculate an amount of energy received from the detection object by the transmission and reception module, and a determination module configured to determine, based on the amount of energy calculated by the calculation module, the contamination state of the detection object.