Process for producing methacrylic acid

In a method of producing methacrylic acid by passing a material gas containing methacrolein, oxygen and water vapor through a reactor packed with a catalyst containing as the main component a compound oxide containing molybdenum and phosphorus, a concentration of methacrolein in the material gas is controlled in a range of 4 to 6.5 vol %, a molar ratio of the water vapor to the methacrolein in the material gas is controlled in a range of 1 to 2, and a space velocity of the material gas to the catalyst-packed layer is controlled in a range of 500 to 750 h −1 , whereby deterioration of the catalyst is effectively inhibited.