To provide an exceptional agent for preventing or treating dementia and schizophrenia, the agent being based on a serotonin 5-HT5A receptor regulating action. The inventors perfected the invention upon discovering that a tetrahydroisoquinoline derivative characterized by having a structure in which an acylguanidine group is bonded to an N atom of a tetrahydroisoquinoline ring, and a ring group is bonded to an unsaturated ring, has a powerful 5-HT5A receptor regulating action as well as an exceptional pharmacological action based thereon； and learning that the tetrahydroisoquinoline derivative is useful as an agent for preventing or treating dementia, schizophrenia, and other conditions.
