A novel therapeutic agent effective for the treatment of intellectual disability or autism is disclosed. The therapeutic agent for intellectual disability or autism contains, as an active component(s), at least one selected from the group consisting of tipifarnib and lonafarnib. Examples of the intellectual disability include memory impairment. Examples of the memory impairment include memory impairment caused by abnormality of the Tsc1 gene and/or Tsc2 gene, and epilepsy-induced memory impairment. Also provided is a method of treating intellectual disability or autism, comprising administering an effective amount of at least one selected from the group consisting of tipifarnib and lonafarnib to a patient with intellectual disability or autism.