A novel process for the preparation of 2-(10,11-dihydro-10-oxodibenzo[b, f]thiepin-2-yl)propionic acid which shows high anti-inflammatory and analgetic action is disclosed. The process starts from a propiophenone derivative having the formula (II): ##STR1## wherein R.sup.1 is hydrogen or a lower alkyl group, which is once converted into a haloacetal compound via a halo-ketone compound, and then converted into the desired dibenzothiepin derivative through a combination of rearrangement, hydrolysis and ring closure in variable sequences.