5-Cyanopyrimidine derivatives were synthesized starting from 5-aminopyrimidine derivatives through the extended Sandmeyer reaction, and then these compounds were hydrolyzed to obtain pyrimidine-5-carboxylic acids. According to this procedure, 5-cyanopyrimidine derivatives and pyrimidine-5-carboxylic acid derivatives have been prepared in 62% and 65% yields, respectively, without any difficulties in removing impurities.