PURPOSE: To obtain the titled compound having high purity, excellent polymerizability and reduced coloring of the formed polymer, by using a specific extracting solvent and treating the titled compound with a hydrazine in obtaining the titled compound by catalytic vapor-phase oxidation of isobutylene, etc.

CONSTITUTION: Isobutylene, tert-butanol, methacrolein, or isobutyl aldehyde is catalytically oxidized in the vapor phase and the resultant methacrylic acid (MAA)-containing reaction product gas is cooled and condensed to give an aqueous solution of the MAA. Light-boiling substances contained in the above- mentioned aqueous solution are then removed by distillation or stripping and the MAA is extracted from the resultant aqueous solution or the MAA using a 6W20C aliphatic chain saturated hydrocarbon as an extracting extracting solvent. The resultant MAA-containing solution is then treated in steps of separating the solvent, the light-boiling substance and heavy substances, redistilling and purifying the resultant product. In the steps, 0.001W1wt%, particularly 0.01W0.5wt% hydrazine, e.g. hydrazine hydrate or phenylhydrazine, is added to any step thereof (preferably the redistillation and purification step).