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): ##STR00001## wherein R represents a cyano group, a carboxyl group or an ester group, R.sup.1 and R.sup.2 each represent a group which does not participate in the reaction, which may have a substituent, and R.sup.1 and R.sup.2 may be combined to each other to form a ring, which comprises subjecting an acetic acid compound represented by the formula (1): ##STR00002## wherein R, R.sup.1 and R.sup.2 have the same meanings as defined above, to decarboxylation in the presence of a metal catalyst.