The object of the invention is to provide a method for producing (meth)acrylic acid or (meth)acrolein from at least one of propylene, propane, isobutylene and (meth)acrolein as a raw material by carrying out its gas phase catalytic oxidation reaction with molecular oxygen or a molecular oxygen-containing gas using a multi-tubular reactor, which is a method that can be operated more safely by feeding a gas prepared in advance by mixing the raw material with molecular oxygen or a molecular oxygen-containing gas safely into the reactor by controlling generation and propagation of fire. The invention is a method for producing (meth)acrylic acid or (meth)acrolein, characterized in that a mixed gas formed by mixing the aforementioned raw material with the aforementioned molecular oxygen or a molecular oxygen-containing gas is fed into the aforementioned reactor via a piping equipped with a flame arrester.