Disclosed is a shell-and-tube reactor that may be used for fixed-bed catalytic partial oxidation, the reactor being characterized by including at least one reaction zone of a first-step reaction zone for mainly producing unsaturated aldehydes and a second-step reaction zone for mainly producing unsaturated acids, wherein at least one reaction zone of the above reaction zones comprises two or more catalytic layers; each of the catalytic layers is packed with a formed product of catalyst that is different in pore density and/or pore size in a catalytically active component; and the pore density and/or pore size is controlled in such a manner that specific surface area of the catalytically active component increases from the inlet of the reactor to the 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 stable use of a catalyst, and to produce unsaturated aldehydes and/or unsaturated fatty acids with high yield.