Disclosed is a shell-and-tube reactor that may be used in a method for producing unsaturated aldehydes and/or unsaturated acids from olefins by means of fixed-bed catalytic partial oxidation. The reactor includes at least one reaction zone of a first-step reaction zone for producing unsaturated aldehydes as a main product and a second-step reaction zone for producing unsaturated acids as a main product, and at least one reaction zone of the reaction zones comprises two or more catalytic layers, each of the catalytic layers being packed with a formed product of catalyst as secondary particles, wherein the secondary particles in each catalytic layer are formed of primary particles of a catalytically active component having a different particle size, and the particle size of primary particles of the catalytically active component is controlled so that it decreases from an inlet of the reactor to an outlet of the reactor. A method for producing unsaturated aldehydes and/or unsaturated fatty acids from olefins using the same reactor is also disclosed. According to the present invention, it is possible to control the temperature efficiently at a hot spot, thereby permitting a catalyst to be used stably, and to produce unsaturated aldehydes and/or unsaturated fatty acids with high yield.