The present invention relates to a method for producing methacrolein and/or methacrylic acid characterized in that the catalyst filling length of a dehydration catalyst layer is 3 to 20% of the catalyst filling length of an oxidation catalyst layer in a method where raw material gas containing gaseous t-butanol is supplied to a fixed-bed multitubular reactor having a dehydration catalyst layer and an oxidation catalyst layer in this order, from the entrance for raw material gas toward the exit, to produce methacrolein and/or methacrylic acid by dehydration reaction and catalytic gas phase oxidation reaction, and the present invention can increase the conventional yield of approximately 80% to 81 to 82% even when a reaction bath temperature is relatively low (approximately 355° C.).