Process for cleaning harmful gas

A process for cleaning a harmful gas, which comprises contacting at a temperature of 10.degree. to 50.degree. C. a gas containing as a harmful component at least one alkoxide compound represented by the chemical formula M(OR).sub.x wherein M is an element selected from the group consisting of silicon, arsenic, phosphorus, boron, tantalum and titanium, OR is an alkoxy group and x is the valency of the element, with a cleaning agent which comprises an alkali metal compound selected from the group consisting of a hydroxide, an oxide, and a carbonate, supported on a metallic oxide comprising cupric oxide and manganese dioxide, wherein the total amount of the cupric oxide and manganese dioxide is at least 60% by weight based on the total amount of the metallic oxide, and wherein said alkali metal compound is present in an amount of 10 to 50 parts by weight based on 100 parts by weight of said metallic oxide, to remove said harmful component from said harmful gas.