To improve productivity by using a solid catalyst with a specified formula for aldonization and/or aldonization-crotonization of aldehyde and/or ketone as a catalyst for producing β-hydroxy and/or α,β-unsaturated carbonyl compounds.

A solid catalyst having the defined formula (in the formula, 0.20≤x≤0.33; n<1) for aldonization and/or aldonization-crofonization of aldehyde and/or ketone is employed as a catalyst for producing β-hydroxy and/or α,β-unsaturated carbonyl compounds from aldehyde and ketone. The n value of the formula is preferably within a range from 0.5 to 0.75 and further preferably set to be 0.81-x or near the value. The solid catalyst is produced by burning hydrotalcite, hydrating the resultant compound oxide Mg1-xAlxO1+X in the absence of CO2, and then washing the hydrated oxide with an insoluble solvent.