Today, either the DSM‒Ⅳ or the ICD‒10 is generally used for forensic purposes, especially for evaluations of mental competency. The use of operational criteria, such as the DSM‒Ⅳ, in forensic settings has some risks. Here, these risks, as well as the advantages of operational criteria and precautions for their use, are discussed. Compared with the DSM‒Ⅳ, the ICD‒10 is preferred because this tool is less likely to complicate evaluations of the mental status of a criminal at the time of the crime when sufficient information is not available to make a diagnosis. The evaluation consists of two steps. The first step, which is based on empirical science, is to provide a psychiatric diagnosis. The second step, which is based on normative science, is to allocate the diagnosis to one of four categories of a forensic frame of reference and to provide useful information for judicial members to make a judgment about the mental competency of the criminal. To standardize evaluations, the use of not only global standard criteria, but also a general rule for the judgment of mental competency within each allocated category is needed.