[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.