A method for producing (meth)acrylic acid or (meth)acrolein, comprising a catalytic gas phase oxidation process for producing (meth)acrylic acid or (meth)acrolein by supplying propylene, propane, or isobutylene and a gas containing molecular oxygen to a reactor which contains a catalyst filled therein, wherein a gas containing molecular oxygen is supplied to the reactor having the catalyst filled therein during shutdown of the catalytic gas phase oxidation process does not lower the reaction activity of the catalyst and the selectivity of target products when re-starting the running after shutdown of the catalytic gas phase oxidation process.