The invention provides a method of gas-phase catalytic oxidation, in particular, a production method of (meth)acrylic acid, which enables stable continuous operation of gas-phase catalytic oxidation over a long term, maintaining high yield and suppressing increase in pressure loss. In the method a fixed bed reactor is used, in which a treating agent for removing organic substance and/or carbides, preferably a treating agent having an adsorption capacity of at least 0.05% by mass, as measured by crotonaldehyde as an indicator of organic substance, is disposed on the upstream side of the gas-phase oxidation catalyst layer in respect of the direction of the gas flow. It is desirable that at least a part of the treating agent is exchanged at a frequency of at least once a year.