Provided is a method for preparing a diol. In the method, a saccharide and hydrogen as raw materials are contacted with a catalyst in water to prepare the diol. The employed catalyst is a composite catalyst comprised of a main catalyst and a cocatalyst, wherein the main catalyst is a water-insoluble acid-resistant alloy; and the cocatalyst is a soluble tungstate and/or soluble tungsten compound. The method uses an acid-resistant, inexpensive and stable alloy needless of a support as a main catalyst, and can guarantee a high yield of the diol in the case where the production cost is relatively low.

The present invention relates to a method for producing an unsaturated aldehyde corresponding to an alkene including partially oxidizing the alkene using a fixed-bed multitubular reactor. The fixed-bed multitubular reactor includes multiple reaction tubes and a reaction bath for adjusting temperature of the multiple reaction tubes. The reaction tube is provided with two or more catalyst layers in a gas flow direction. When a reaction bath temperature at which a yield of the unsaturated aldehyde is the highest is defined as A ( C.), and reaction bath temperatures at which the yield is 1.0% lower than the highest value are defined as A1 ( C.) and A2 ( C.), following formulae (1) and (2) hold: A1<A<A2 (1); and (A2A1)10 (2).