The present invention discloses a process for preparing a nitrile compound, a carboxylic acid compound or a carboxylic acid ester compound represented by the formula (2): wherein R represents a cyano group, a carboxyl group or an ester group, R1 and R2 each represent a group which does not participate in the reaction, which may have a substituent, and R1 and R2 may be combined to each other to form a ring, which comprises subjecting an acetic acid compound represented by the formula (1): wherein R, R1 and R2 have the same meanings as defined above, to decarboxylation in the presence of a metal catalyst.