A catalyst for gas phase oxidation of methylbenzenes in the presence of molecular oxygen to produce corresponding aromatic aldehydes, a method for preparing the catalyst, and a method for producing aromatic aldehydes from methylbenzenes by using the catalyst. The catalyst comprises a compound represented by the following formula (1): W.sub.aX.sub.bY.sub.cO.sub.x (1)wherein W represents a tungsten atom, X represents one or more alkali metals selected from the group consisting of Li, Na, K, Rb, and Cs, Y represents one or more elements selected from the group consisting of Fe, Co, Ni, Cu, Mn, Re, Cr, V, Nb, Ti, Zr, Zn, Cd, Y, La, Ce, B, Al, Sn, Mg, Ca, Sr, and Ba, O stands for an oxygen atom, and the ratio of a:b:c is 12:0.001.about.1:0.about.5.