The present invention relates to a synthetic lubricating oil comprising cyclic ketals or cyclic acetals obtained by a reaction between one or more polyhydric alcohols having an even number of hydroxyl groups of not less than 4 and not more than 10 and one or more specific carbonyl compounds, or one or more ketals or acetals which are reactive derivatives of said carbonyl compounds; a working fluid composition for a refrigerating machine comprising a hydrofluorocarbon and a refrigeration oil containing the cyclic ketals or cyclic acetals; and novel compounds of the above cyclic acetals and a production method thereof. The present invention provides a synthetic lubricating oil which shows a good thermal stability, a good oxidation resistance, no carboxylic acid formation due to hydrolysis and a low hygroscopicity and is inexpensive as well. Also, a working fluid composition for a refrigerating machine which shows a good insulating property, favorable hygroscopicity, and no carboxylic acid formation due to hydrolysis as well as being inexpensive can be provided.