There are disclosed a process for cleaning a harmful gas which comprises bringing the harmful gas containing as a harmful component, an organosilicon compound represented by the general formula: CH2CH—SiR3, CH2CH—Si(OR)3, CH2CHCH2—SiR3 or CH2CHCH2—Si(OR)3, wherein R indicates a saturated hydrocarbon group or an aromatic compound group, into contact with a cleaning agent comprising activated carbon adhesively incorporated with at least one species selected from the group consisting of bromine, iodine, a metal bromide and a metal iodide in which the metal is exemplified by copper, lithium, sodium, potassium, magnesium, calcium, strontium, manganese, iron, cobalt, nickel, zinc, aluminum and tin; and a cleaning agent comprising the same. The cleaning process and the cleaning agent enable to practically clean a harmful gas which is exhausted from a semiconductor manufacturing process and the like by the use of a dry cleaning process.