In recent years, offender treatment programs based on cognitive behavioral therapy have been introduced in Japan. I would like to share the result of survey-based studies, which focused on offenders’ irrational beliefs in order to clarify offenders’ cognitive tendencies in Japan, as a reference for more effective treatment for offenders. “Survey Study I” was conducted on 128 inmates whose criminal tendencies were progressed; based on that result, an irrational beliefs checklist was formulated, which was used in the “Survey Study II”. “Survey Study II” was conducted on 241 inmates whose criminal tendencies were progressed. The study (i) collected verbalized irrational beliefs, then (ii) analyzed and examined the verbalized irrational beliefs, and then extracted the “abstracted irrational beliefs” with high commonality, and finally (iii) examined the relation between the inmates’ attributes and their irrational beliefs. The results indicate that the subjects tend to have irrational beliefs which may impede introspection and may hinder rehabilitation by denying, avoiding and shifting self-responsibility. Moreover, the results suggest that when a subject had a sense of powerlessness and a feeling of fate, they tend to have irrational beliefs which may cause desperation and hasten re-offending.