[Problem] To provide a novel catalyst for selectively reducing
propionaldehyde from a starting-material
acrolein in order to produce an
acrolein and / or
acrylic acid and / or
acrylonitrile and / or methylmercaptopropionaldehyde that has a low-concentration of
propionaldehyde and / or propionic acid and / or
propionitrile. In particular, to provide a catalyst used to selectively reduce
propionaldehyde-containing
acrolein. [Solution] A catalyst used for selectively reducing propionaldehyde from propionaldehyde-containing acrolein, said catalyst including Mo as the essential component, and further containing at least one element selected from P, Si, W, Ti, Zr, V, Nb, Ta, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, In, Tl, Sn, Ag, As, Ge, B, Bi, La, Ba, Sb, Te, Ce, Pb, Mg, K, Rb, Cs and Al, and preferably from P, Si, W, Ti, Cr, Mn, Fe, Co, Ni, Zn, Sn, Bi, Sb, Ce, Mg, Cs and K.