Mice in which OCT3 expression is suppressed were successfully constructed by administering antisense against OCT3 gene into the brain. Mice in which OCT3 expression is suppressed display phenotypes related to mental disorders such as anti-depression and anti-anxiety, and therefore the mice can be applied to the screening of therapeutic agents for mental disorders. In addition, substances suppressing the OCT3 expression or function were shown to be in fact effective as therapeutic agents for depression and anxiety neuroses.