A process for preparing trichloroacetic acid comprises the step of bringing functional water capable of partial oxidizing tetrachloroethylene under irradiation with light or aerated functional water gas which is produced by aerating the above functional water and has a capability similar to that of the above functional water into contact with tetrachloroethylene under irradiation with light. And an apparatus for preparing trichloroacetic acid comprises a closable container as a means for bringing functional water capable of partial oxidizing tetrachloroethylene under irradiation with light or aerated functional water gas which is produced by aerating the above functional water and has a capability similar to that of the above functional water into contact with tetrachloroethylene, and means for irradiating the above contact mixture with light.