Provided are an inhibitor for RuO4 gas generation used in a manufacturing process of a semiconductor element, that inhibits a RuO4 gas generated when a semiconductor wafer containing ruthenium and a treatment liquid are brought into contact, and a method for inhibiting the RuO4 gas. Specifically, provided is an inhibitor for RuO4 gas generation for inhibiting a RuO4 gas generated when a semiconductor wafer containing ruthenium and a treatment liquid are brought into contact in semiconductor formation steps, wherein the inhibitor includes an onium salt consisting of an onium ion and a bromine-containing ion. Also provided is a method for inhibiting RuO4 gas generation by adding the inhibitor to a ruthenium treatment liquid or a ruthenium-containing liquid used in semiconductor formation steps.