PURPOSE:To obtain an arylethylene glycol or arylacetaldehyde in good yield by efficiently hydrolyzing or isomerizing the epoxy ring of an arylethylene oxide in a specific solvent using niobic acid as a catalyst. CONSTITUTION:An arylethylene oxide expressed by formula I (R to R are substituent group, such as halogen, alkyl or alkoxy, insert to reaction) is treated with niobic acid calcined preferably at 100-500 deg.C temperature in a solvent selected from (a) water or hydrous solvents and (b) organic solvents to suppress side reaction and afford (A) an arylethylene glycol expressed by formula II or (B) an arylacetaldehyde expressed by formula III under mild conditions using the readily handleable catalyst capable of exhibiting excellent catalysis on ring opening, hydrolytic reaction and isomerizing reaction of the epoxy ring in the aforementioned raw material with hardly any deterioration in activity even in use for a long period.