A process for producing amide derivatives represented by general formula (XV) or enantiomers thereof, novel intermediate compounds useful for producing these compounds, etc., and a process for producing the same, wherein each symbol is as defined in the specification. Compared with the conventional processes, the above process can be very easily effected and enables an efficient production of the compounds (XV), i.e., those involving the compounds (XVI) having an HIV protease inhibitory effect in high yields. The novel intermediates are highly advantageous as intermediates for producing not only the above compounds (XVI) but also those which are useful as an X-ray contrast medium, etc.